Use install_venv_common.py from oslo.

This syncs install_venv_common.py from oslo and reworks the
tools/install_venv.py script to use the new library.

Change-Id: I020eae1ec9e3a6aae0ecc7825c0ef723ab247f39
Merge "Use install_venv_common.py from oslo."
Merge remote-tracking branch 'citrix/master' into build-temp