Updated openstack/openstack
Project: openstack/glance  b46784e7d583e206593f518a915609920d96db9f

Add specific docs build option to tox

Add doc venv to align with other OpenStack projects (such as keystone,
heat and nova) who are using 'docs' for their doc builds.

Change-Id: I8bccbf9c6b1eae5de9f74396fd5159662ad04149