Replace assertEqual(None, *) with assertIsNone in tests

Replace assertEqual(None, *) with assertIsNone in tests to
have more clear messages in case of failure.

Change-Id: Ice018dfd3e48326ce9cac58342d03979559b9e82
diff --git a/neutron/tests/tempest/api/test_floating_ips.py b/neutron/tests/tempest/api/test_floating_ips.py
index bafa54c..6e722db 100644
--- a/neutron/tests/tempest/api/test_floating_ips.py
+++ b/neutron/tests/tempest/api/test_floating_ips.py
@@ -100,4 +100,4 @@
         # disassociate
         body = self.client.update_floatingip(body['floatingip']['id'],
                                              port_id=None)
-        self.assertEqual(None, body['floatingip']['port_id'])
+        self.assertIsNone(body['floatingip']['port_id'])