auth_unittests tries to skip some tests, but unittest doesn't support that

Per jbryce on https://code.launchpad.net/~ed-leafe/nova/remove-redis/+merge/44074: "On lines 338-342 of /nova/tests/auth_unittest.py the exception handling is now superfluous and the self.skip is unnecessary as well. 'skip' was a property available from the TrialTestCase class that isn't used with PyUnit TestCase. This is minor but it will keep future code readers from wondering what that block is supposed to be doing."