[wip] RAX_ to RS_

Rename the environment variables read by `rackspace.AuthOptionsFromEnv()` to `RS_` instead of `RAX_`. I've also added a fallback to `OS_`-prefixed options if those are present, and changed the acceptance tests to *not* do that, because otherwise your `OS_AUTH_URL` will make all of the Rackspace tests run against your devstack server!
Do we want to automatically fall-back to Openstack environment variables? It may be clearer if we just let the user know to set the `RS_` variables.