Add docs to tox
Add docs to tox

In order to know about nova-docker, we can use the command
'tox -e docs' to build nova-docker documents conveniently.

Change-Id: Ib172c71c7cb94f83a497112496787a5344ea9a47

Juniper/nova-docker,stackforge/nova-docker,Juniper/nova-docker,stackforge/nova-docker