Add SkipCopyPythonClients for building pulsar docker image (apache#…

…3798)

*Motivation*

Currently we use `skipBuildPythonClients` for controlling both building
python clients and copying python clients. So we have to disable or enable
all together.

This introduces a problem for building pulsar docker image
in a kubernetes environment (either docker-in-docker or docker-out-of-docker).

An alternative approach for building pulsar docker image in a k8s environment
will be:

- build python client for py27 using `apachepulsar/pulsar-build:manylinux-cp27-cp27mu`
  in a container
- build python client for py35 using `apachepulsar/pulsar-build:manylinux-cp35-cp35m`
  in a container
- build pulsar docker image

This requires we separating building python clients from copying python clients.

*Modifications*

- introduce a new property `skipCopyPythonClients` to skip copying python clients
- `skipBuildPythonClients` is only used for skipping building python clients

So the above build process can be done by specifying `-DskipCopyPythonClients`

docker/pulsar/pom.xml

@@ -97,7 +97,7 @@
                   <goal>run</goal>
                 </goals>
                 <configuration>
-                  <skip>${skipBuildPythonClient}</skip>
+                  <skip>${skipCopyPythonClients}</skip>
                   <tasks>
                     <echo>copy python wheel file</echo>
                     <mkdir dir="${basedir}/target/python-client"/>