Replace the usage of 'admin_manager' with 'os_admin'

In tempest, alias 'admin_manager' has been moved to 'os_admin'
in version Pike, and it will be removed in version Queens [1].

[1] https://review.openstack.org/#/c/467852/

Change-Id: I6d51cfcf2a5c931b5af2e32fa2ef153181fbca88
diff --git a/patrole_tempest_plugin/tests/unit/test_rbac_utils.py b/patrole_tempest_plugin/tests/unit/test_rbac_utils.py
index 0f38b1e..85b1c39 100644
--- a/patrole_tempest_plugin/tests/unit/test_rbac_utils.py
+++ b/patrole_tempest_plugin/tests/unit/test_rbac_utils.py
@@ -72,7 +72,7 @@
         self.addCleanup(mock.patch.stopall)
 
     def _mock_list_user_roles_on_project(self, return_value):
-        self.mock_test_obj.admin_manager = mock.Mock(
+        self.mock_test_obj.os_admin = mock.Mock(
             **{'roles_client.list_user_roles_on_project.'
                'return_value': {'roles': [{'id': return_value}]}})