Updated openstack/openstack
Project: openstack/nova  d01037018d335b325c9cc30a77367351a966c138

Add specific doc build option to tox

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

Change-Id: I37771db339d864b2b938d1c02d15d38b07093493