Cleans up the code around exception handling and logging
when first authenticating (which often returns an unscoped token).
There's no need to be logging an exception on an expected empty
catalog and moreover the except block was a bare "except" which
could mask other errors.
Fixes bug 1070493
Change-Id: I5e791e95ce3f9ab77723a7f4698cb11b169dacfb
Bug: 1028799
No traceback, if service is unavailable; error out gracefully.
* keystoneclient/exceptions.py
ServiceUnavailable: New class to handle 503 status code.
Add the new class to list of handled exceptions.
Change-Id: I39a8ac594ef922d682731a926be26c8b6f648f9d
