Using assertIsNone() instead of assertEqual(None)

Following OpenStack Style Guidelines[1]:
[H203] Unit test assertions tend to give better messages for more
specific assertions. As a result, assertIsNone(...) is preferred
over assertEqual(None, ...) and assertIs(..,None)

[1] http://docs.openstack.org/developer/hacking/#unit-tests-and-assertraises

Change-Id: Iae83c4360336cf07b6045615ff38933f52844499

ceilometerclient/tests/unit/v2/test_alarms.py

@@ -400,7 +400,7 @@ class AlarmManagerTest(testtools.TestCase):
             alarm = self.mgr.update(alarm_id='unk-alarm-id', **UPDATE_ALARM)
         except exc.CommandError:
             pass
-        self.assertEqual(alarm, None)
+        self.assertIsNone(alarm)
 
     def test_delete_from_alarm_class(self):
         alarm = self.mgr.get(alarm_id='alarm-id')