Sync latest install_venv_common.py
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

Change-Id: I05b69f043b276d5e6cd2df5d90bc679df6373383

emonty/oslo-hacking,hyakuhei/cleantox,hyakuhei/cleantox,emonty/oslo-hacking,zancas/hacking,openstack-dev/hacking,zancas/hacking,openstack-dev/hacking