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 there doc builds.

Change-Id: I0094130224ec2667cdd76f497edf595f09a86d12

CiscoSystems/networking-odl,mestery/x-stackforge-odl,openstack/networking-odl,schoksey/networking-odl,schoksey/networking-odl,CiscoSystems/networking-odl,flavio-fernandes/networking-odl,FedericoRessi/networking-odl,openstack/networking-odl,openstack/networking-odl,FedericoRessi/networking-odl,flavio-fernandes/networking-odl