fix per using supernova.supernova.SuperNova.get_novaclient()

- novaclient.Client doesn't like having the rax_auth entry in the creds dictionary
- novaclient.Client() requires either (i) a version be specified, or (ii) the correct version be imported; specifying the version seems to be easier to maintain than changing the import statement. Opted for the latest version (3), but doesn't really matter whether it's '1_1', '2', or '3' (see novaclient's novaclient.client.py).