Handle "503 Service Unavailable" exception.
Handle "503 Service Unavailable" exception.

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

sdpp/python-keystoneclient,darren-wang/ksc,ntt-sic/python-keystoneclient,ntt-sic/python-keystoneclient,ging/python-keystoneclient,klmitch/python-keystoneclient,jamielennox/python-keystoneclient,klmitch/python-keystoneclient,alexpilotti/python-keystoneclient,alexpilotti/python-keystoneclient,citrix-openstack-build/python-keystoneclient,metacloud/python-keystoneclient,sdpp/python-keystoneclient,magic0704/python-keystoneclient,Mercador/python-keystoneclient,Mercador/python-keystoneclient,citrix-openstack-build/python-keystoneclient,citrix-openstack-build/python-keystoneclient,ging/python-keystoneclient,darren-wang/ksc,metacloud/python-keystoneclient,ntt-sic/python-keystoneclient,magic0704/python-keystoneclient,jamielennox/python-keystoneclient,jamielennox/python-keystoneclient