Sync latest install_venv_common.py with olso

This patch syncs with oslo change set of
> c78b2ab Avoid using cfg in install_venv_common

So that oslo-config isn't needed to run install_venv

Change-Id: Ia7ee923a53e09c0c6a525294ec4a5dc9d31c3bed
Fixes: bug #1131009
Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Merge "Sync latest install_venv_common.py with olso"