Add support for headnode base-image selection

The deployment tests now require TWO base-images, named base-headnode and
base-headnode-2
Merge branch 'devel' into multiple-base-imgs
Merge pull request #294 from gsilvis/multiple-base-imgs

Add support for headnode base-image selection
Implement null auth backend

Authentication/authorization backends belong in haas/auth_mechanisms .  It's
possible that the API of that plugin will have to be expanded to cover what
the CLI needs to do.