Use --no-site-package for services venv's (-nova)

The --system-site-packages option essentially removes the benefits of
virtualenv, nova is a special case because it depends on python-libvirt
which can't be installed via pip.

Change-Id: Ifc3580d4b59a5f1e97de77c0c4a875b123208100
Merge "Use --no-site-package for services venv's (-nova)"