Use install_venv_common.py from oslo.
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: I46bbf7d656e6cb9660e37af2d3ce96150e18e7fe

openstack/networking-cisco,Gitweijie/first_project,CiscoSystems/networking-cisco,Tehsmash/networking-cisco-nexus,nikolay-fedotov/networking-cisco,cisco-openstack/networking-cisco,cisco-openstack/networking-cisco,Tehsmash/networking-cisco,noironetworks/networking-cisco,CiscoSystems/networking-cisco,openstack/networking-cisco,Tehsmash/networking-cisco,hareeshpc/networking-cisco,noironetworks/networking-cisco,Tehsmash/networking-cisco-apic,Gitweijie/first_project,hareeshpc/networking-cisco