Remove python2 from the pulsar images build (apache#7936)

The pulsar docker and pulsar standalone docker image build failed by:
```
Processing ./pulsar-client/pulsar_client-2.6.0-cp27-cp27mu-manylinux1_x86_64.whl
2020-08-25T02:23:28.9567844Z [INFO] Collecting fastavro
2020-08-25T02:23:29.0080316Z [INFO]   Downloading fastavro-1.0.0.tar.gz (658 kB)
2020-08-25T02:23:29.3108074Z [INFO]     ERROR: Command errored out with exit status 1:
2020-08-25T02:23:29.3109489Z [INFO]      command: /usr/bin/python2.7 -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-install-jvZueR/fastavro/setup.py'"'"'; __file__='"'"'/tmp/pip-install-jvZueR/fastavro/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' egg_info --egg-base /tmp/pip-pip-egg-info-iH1mMy
2020-08-25T02:23:29.3110061Z [INFO]          cwd: /tmp/pip-install-jvZueR/fastavro/
2020-08-25T02:23:29.3110303Z [INFO]     Complete output (5 lines):
2020-08-25T02:23:29.3110520Z [INFO]     Traceback (most recent call last):
2020-08-25T02:23:29.3110741Z [INFO]       File "<string>", line 1, in <module>
2020-08-25T02:23:29.3111190Z [INFO]       File "/tmp/pip-install-jvZueR/fastavro/setup.py", line 58, in <module>
2020-08-25T02:23:29.3111437Z [INFO]         if sys.implementation.name != "pypy":
2020-08-25T02:23:29.3111886Z [INFO]     AttributeError: 'module' object has no attribute 'implementation'
```

I remove the python2.7 and using the python3 to build images.

* Remove python2 in images

* Remove python2 stuff

* Update the default python version

dashboard/Dockerfile

@@ -25,7 +25,7 @@ RUN bash -c "echo deb http://apt.postgresql.org/pub/repos/apt/ stretch-pgdg main
 RUN wget --quiet -O - https://www.postgresql.org/media/keys/ACCC4CF8.asc | apt-key add -
 
 RUN apt-get update
-RUN apt-get -y install postgresql-11 postgresql-contrib libpq-dev python sudo nginx supervisor
+RUN apt-get -y install postgresql-11 postgresql-contrib libpq-dev sudo nginx supervisor
 
 # Postgres configuration
 COPY conf/postgresql.conf /etc/postgresql/11/main/

docker/pulsar-standalone/Dockerfile

@@ -29,7 +29,10 @@ FROM openjdk:8-jdk
 # Note that the libpq-dev package is needed here in order to install
 # the required python psycopg2 package (for postgresql) later
 RUN apt-get update \
-    && apt-get -y install python2.7 python2.7-dev python-pip postgresql sudo nginx supervisor libpq-dev
+    && apt-get -y install python3.7 python3.7-dev python3-pip postgresql sudo nginx supervisor libpq-dev
+
+RUN update-alternatives --install /usr/bin/python python /usr/bin/python3 10
+RUN ln -s /usr/bin/pip3 /usr/bin/pip
 
 # Postgres configuration
 COPY --from=dashboard /etc/postgresql/11/main/postgresql.conf /etc/postgresql/11/main/postgresql.conf
@@ -59,15 +62,17 @@ COPY --from=dashboard /pulsar/django /pulsar/django
 COPY --from=dashboard /pulsar/requirements.txt /pulsar/django
 COPY --from=dashboard /pulsar/conf/* /pulsar/conf/
 
+RUN pip -V
+
 # Python dependencies
-RUN pip2 install -r /pulsar/django/requirements.txt
+RUN pip install -r /pulsar/django/requirements.txt
 
 # Copy web-app sources
 # Setup database and create tables
 COPY --from=dashboard /pulsar/init-postgres.sh /pulsar/django/init-postgres.sh
 RUN mkdir /data
 RUN /pulsar/django/init-postgres.sh
-RUN sudo -u postgres /etc/init.d/postgresql stop
+RUN sudo -u postgres /etc/init.d/postgresql stop 
 # Add postgresql to supervisord. Redirect logs to stdout
 RUN echo "\n[program:postgresql]\n\
 command = /etc/init.d/postgresql start\n\
@@ -78,9 +83,10 @@ redirect_stderr=true" >> /etc/supervisor/conf.d/supervisor-app.conf
 # Collect all static files needed by Django in a
 # single place. Needed to run the app outside the
 # Django test web server
-RUN cd /pulsar/django && ./manage.py collectstatic --no-input
+RUN cd /pulsar/django && python manage.py collectstatic --no-input
 
 ENV SERVICE_URL http://127.0.0.1:8080
 EXPOSE 80 8080 6650
 
 CMD ["supervisord", "-n"]
+

docker/pulsar/Dockerfile

@@ -31,7 +31,6 @@ COPY scripts/generate-zookeeper-config.sh /pulsar/bin
 COPY scripts/pulsar-zookeeper-ruok.sh /pulsar/bin
 COPY scripts/watch-znode.py /pulsar/bin
 COPY scripts/set_python_version.sh /pulsar/bin
-COPY scripts/install-pulsar-client-27.sh /pulsar/bin
 COPY scripts/install-pulsar-client-37.sh /pulsar/bin
 
 
@@ -43,17 +42,17 @@ FROM openjdk:8-jdk-slim
 # Install some utilities
 RUN apt-get update \
      && apt-get install -y netcat dnsutils less procps iputils-ping \
-                 python2.7 python2.7-dev python-setuptools python-yaml python-kazoo \
                  python3.7 python3.7-dev python3-setuptools python3-yaml python3-kazoo \
                  libreadline-gplv2-dev libncursesw5-dev libssl-dev libsqlite3-dev tk-dev libgdbm-dev libc6-dev libbz2-dev \
                  curl \
      && apt-get clean \
      && rm -rf /var/lib/apt/lists/*
 
 RUN curl https://bootstrap.pypa.io/get-pip.py -o get-pip.py
-RUN python2.7 get-pip.py
 RUN python3.7 get-pip.py
 
+RUN update-alternatives --install /usr/bin/python python /usr/bin/python3 10
+
 ADD target/python-client/ /pulsar/pulsar-client
 ADD target/cpp-client/ /pulsar/cpp-client
 RUN echo networkaddress.cache.ttl=1 >> $JAVA_HOME/jre/lib/security/java.security
@@ -70,6 +69,5 @@ ENV PULSAR_ROOT_LOGGER=INFO,CONSOLE
 COPY --from=pulsar /pulsar /pulsar
 WORKDIR /pulsar
 
-RUN /pulsar/bin/install-pulsar-client-27.sh
 RUN /pulsar/bin/install-pulsar-client-37.sh
 

docker/pulsar/pom.xml

@@ -52,22 +52,6 @@
             <artifactId>exec-maven-plugin</artifactId>
             <version>${exec-maven-plugin.version}</version>
             <executions>
-              <execution>
-                <id>build-pulsar-clients-python-27</id>
-                <phase>compile</phase>
-                <goals>
-                  <goal>exec</goal>
-                </goals>
-                <configuration>
-                  <skip>${skipBuildPythonClient}</skip>
-                  <workingDirectory>${project.basedir}/target</workingDirectory>
-                  <executable>${project.basedir}/../../pulsar-client-cpp/docker/build-wheels.sh</executable>
-                  <arguments>
-                    <!-- build python 2.7 -->
-                    <argument>2.7 cp27-cp27mu</argument>
-                  </arguments>
-                </configuration>
-              </execution>
               <execution>
                 <id>build-pulsar-clients-python-35</id>
                 <phase>compile</phase>