Add venv testenv to tox.ini.

The automagic pypi publishing of releases through jenkins.openstack.org
depends on tox having a testenv called venv. Add this new testenv so
that git-review can be released automagically.

Change-Id: I89d78a22c13f94b5af3cda1115c8068424d42b34
Merge "Add venv testenv to tox.ini."