Replace assertEqual(None, *) with assertIsNone(*)

This patch is going to replace assertEqual(None, *) with
assertIsNone(*) in unit test code to have more clear messages
in case of failure.

Change-Id: I6f85498347e8fc7cad5ea7afb832b9acda7daafc

Author: Xi Yang

openstackclient/tests/common/test_parseractions.py

@@ -101,7 +101,7 @@ def test_error_values(self):
         # There should be no red or blue
         expect = {'green': '100%'}
         self.assertDictEqual(expect, actual)
-        self.assertEqual(None, failhere)
+        self.assertIsNone(failhere)
 
 
 class TestNonNegativeAction(utils.TestCase):