Make the checks of identity status code strict

Some identity API tests check response status codes like:

 self.assertTrue(resp['status'].startswith('2'))

This way cannot detect the status code changes and that means
Tempest cannot block the backward incompatibility changes for
Keystone.

This patch makes these checks strict by specifying particular
status codes.

Closes-Bug: #1293982

Change-Id: Icab5ff57aaa06e313da209f87ced4c2135ab5343
Merge "Make the checks of identity status code strict"