Allow usage of api_url and auth_token to bypass auth_url

We have a use case where we need to use an existing auth token, rather than hit the auth_url on each request.  This is exposed in the underlying rackspace-monitoring library but is not made available to rackspace-monitoring-cli.
Hi phoebesimon,

The user needs to enter:

username AND api_key

OR

auth_token AND api_url