Sync latest install_venv_common.py

Syncs:

c78b2ab Avoid using cfg in install_venv_common

So that people don't need oslo-config installed to run install_venv

Fixes bug 1130853

Change-Id: I9dfc8d8417455ff22a0d0e911c762983952c75ee
Merge "Sync latest install_venv_common.py"