Create rally deployment from OS_* environment variables

This saves users from creating dummy config json files when they already
have an openrc for use with the other openstack clients.

Change-Id: I487af60e0c6a5eadfc12f04fbba9c4a4186740e0
Trello: https://trello.com/c/5ZkMb9zL
Merge "Create rally deployment from OS_* environment variables"