Add 'docs' tox environment
Add 'docs' tox environment

To make it easy to build docs with 'tox -e docs'.

Change-Id: Idd47c617196b25539b6b1fc88fac0a45b7fc0911

dims/oslo.messaging,zhurongze/oslo.messaging,dims/oslo.messaging,eayunstack/oslo.messaging,hkumarmk/oslo.messaging,redhat-openstack/oslo.messaging,stevei101/oslo.messaging,apporc/oslo.messaging,hkumarmk/oslo.messaging,apporc/oslo.messaging,dukhlov/oslo.messaging,viggates/oslo.messaging,magic0704/oslo.messaging,ozamiatin/oslo.messaging,redhat-openstack/oslo.messaging,zhurongze/oslo.messaging,ozamiatin/oslo.messaging,stevei101/oslo.messaging,magic0704/oslo.messaging,isyippee/oslo.messaging,dukhlov/oslo.messaging,isyippee/oslo.messaging