Update tox.ini to use new features

Per this email message from Monty:

http://lists.openstack.org/pipermail/openstack-dev/2013-September/015495.html

We need to duplicate the changes from this review:

https://review.openstack.org/47848

Change-Id: I82b3cc16e554d21fbb642629f8d021c8b1e50b87