Add basic sphinx docs setup with some rudimentary information
on how to use gabbi-tempest. This isn't perfect but provides
the basics for future improvement.
A docs template is added to .zuul.yaml.
Change-Id: If2631d92366866ba6bd149909ece17734931ae91
The name follows the patterns defined by zuul docs.
The Depends-On is to a necessary Tempest change for
handling the enviornment variable.
The gabbigts in gate/gabbits talk to keystone and nova
to simply confirm that the tests can run.
Depends-On: https://review.openstack.org/607507
Change-Id: I7a9efefb28e23bf47951617cbe08f177235688d6