Add venv environment to tox
Add venv environment to tox

Some of the stackforge jobs use an environment "venv" to run
commands, like packaging a release.

Change-Id: I3453aaa0c7201d780301447936d26aca8ecfb91f

mandeepdhami/stevedore,JioCloud/stevedore,varunarya10/stevedore,nelsnelson/stevedore,citrix-openstack-build/stevedore,JioCloud/stevedore,mandeepdhami/stevedore,citrix-openstack-build/stevedore,openstack/stevedore,varunarya10/stevedore,nelsnelson/stevedore