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.
just making sure I'm understanding this boolean logic correctly, the user must enter:
username && (auth token || api url) 
or
api_key && (auth token || api url)

The error message should probably say `and` in this last line instead of `or` to make it clearer that you need at least one option from each pair