Remove stubs in favor of mock in test_policy

This patch converts tests_policy to use mock instead of stubs. I was
stealing some tests from nova to another openstack project and made this
change there so I figured nova would benefit from it too.

Change-Id: I6d086795c559ac8f04028d820e8e88bfd5305426
Merge "Remove stubs in favor of mock in test_policy"