Updated openstack/openstack
Project: openstack/cliff  26838213e16bd4000f9d3d46e7b03c155453506d

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: I35e3142a0e21760136ea31a630f67a0df39bf720