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: I8bccbf9c6b1eae5de9f74396fd5159662ad04149

openstack/glance,wkoathp/glance,sigmavirus24/glance,akash1808/glance,scripnichenko/glance,ozamiatin/glance,jumpstarter-io/glance,darren-wang/gl,takeshineshiro/glance,rajalokan/glance,saeki-masaki/glance,kfwang/Glance-OVA-OVF,ozamiatin/glance,vuntz/glance,sigmavirus24/glance,stevelle/glance,jumpstarter-io/glance,akash1808/glance,vuntz/glance,paramite/glance,paramite/glance,darren-wang/gl,JioCloud/glance,rajalokan/glance,stevelle/glance,wkoathp/glance,takeshineshiro/glance,dims/glance,kfwang/Glance-OVA-OVF,JioCloud/glance,redhat-openstack/glance,dims/glance,openstack/glance,openstack/glance,klmitch/glance,klmitch/glance,redhat-openstack/glance,saeki-masaki/glance,scripnichenko/glance