RestClient:keystone_auth hides requests and errors

RestClient::keystone_auth did not logged requests/responses and instead
 of reporting unexpected status codes it silently returned None.
(Which causes 'NoneType is not iterrable' for example for HTTP 500)

Also removed setting of unused variable tenant_id.

Change-Id: I60e074a1180aca89006cc6c6345a2288b125540f
Merge "RestClient:keystone_auth hides requests and errors"