Recent virtualenv review cleanups

This is a follow-on to comments in https://review.openstack.org/156356
and https://review.openstack.org/#/c/151513/

* Remove work-around for /var/cache/pip
* Remove WHEELHOUSE setting in tools/build_wheels.sh and use the pip
  default directory '<cwd>/wheelhouse'
* Remove bogus MySQL-python install
* Removed unused bits and clean up pip commands in from tools/build_venvs.sh

Closes-Bug: #1423720
Change-Id: I0283b0dff9146b1b63bd821358505a93566270c6

lib/databases/postgresql

@@ -102,7 +102,7 @@ EOF
     fi
 
     # Install Python client module
-    pip_install MySQL-python psycopg2
+    pip_install psycopg2
 }
 
 function database_connection_url_postgresql {