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: Ia271de047cc80e462fa0850cd8d626d2da777dff
Fixes "is not" usage

Fixes bug #1110987

Change-Id: Iafa9fc548deaf03c229cc9144d85e6c68b3101be
Merge "Use install_venv_common.py from oslo."
Merge "Fixes "is not" usage"