Updated openstack/openstack
Project: openstack-dev/devstack  56eda9b06d449b7c05e7535edb9dbd712203ae70

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