Add docs environment to tox.ini

As per the governance resolution https://review.openstack.org/119875,
projects should have a docs environment in their tox.ini testing
interface. Add one.

Change-Id: Ib3548bc23713e120089ae29b5c4e851de48cf8d3
(cherry picked from commit 81fa6c7b4bd5a1d57cc095395b9d2ba3eeb6c20f)