Add specific docs build option to tox
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: Ib26cdac7142e9ec9efc231a578c14bfc7936ae61

vponomaryov/manila,scality/manila,weiting-chen/manila,NetApp/manila,jcsp/manila,bswartz/manila,redhat-openstack/manila,openstack/manila,jcsp/manila,scality/manila,bswartz/manila,sajuptpm/manila,vponomaryov/manila,NetApp/manila,sajuptpm/manila,redhat-openstack/manila,openstack/manila,weiting-chen/manila