[AIRFLOW-941] Use defined parameters for psycopg2

This works around
psycopg/psycopg2#517 .

Closes apache#2126 from bolkedebruin/AIRFLOW-941

airflow/hooks/postgres_hook.py

@@ -36,10 +36,17 @@ def get_conn(self):
         conn = self.get_connection(self.postgres_conn_id)
         conn_args = dict(
             host=conn.host,
-            user=conn.login,
-            password=conn.password,
-            dbname=self.schema or conn.schema,
-            port=conn.port)
+            dbname=self.schema or conn.schema)
+        # work around for https://github.com/psycopg/psycopg2/issues/517
+        # todo: remove when psycopg2 2.7.1 is released
+        # https://issues.apache.org/jira/browse/AIRFLOW-945
+        if conn.port:
+            conn_args['port'] = conn.port
+        if conn.login:
+            conn_args['user'] = conn.login
+        if conn.password:
+            conn_args['password'] = conn.password
+
         # check for ssl parameters in conn.extra
         for arg_name, arg_val in conn.extra_dejson.items():
             if arg_name in ['sslmode', 'sslcert', 'sslkey', 'sslrootcert', 'sslcrl', 'application_name']: