Modified into a more appropriate function

If the function includes none parameters, AssertIsNone() is more
appropriate than assertEqual()

Change-Id: I4155133694ca024c717209a9cb7cd6fd11d50f83

tempest/api/network/test_networks.py

@@ -559,7 +559,7 @@ class NetworksIpV6Test(NetworksTest):
         # Verifies Subnet GW is set in IPv6
         self.assertEqual(subnet1['gateway_ip'], ipv6_gateway)
         # Verifies Subnet GW is None in IPv4
-        self.assertEqual(subnet2['gateway_ip'], None)
+        self.assertIsNone(subnet2['gateway_ip'])
         # Verifies all 2 subnets in the same network
         body = self.subnets_client.list_subnets()
         subnets = [sub['id'] for sub in body['subnets']

tempest/tests/lib/test_auth.py

@@ -244,7 +244,7 @@ class TestKeystoneV2AuthProvider(BaseAuthTestsSetUp):
         # The original headers where empty
         self.assertNotEqual(url, self.target_url)
         self.assertIsNone(headers)
-        self.assertEqual(body, None)
+        self.assertIsNone(body)
 
     def _test_request_with_alt_part_without_alt_data_no_change(self, body):
         """Test empty alternate auth data with no effect

tempest/tests/negative/test_negative_generators.py

@@ -146,5 +146,5 @@ class TestNegativeNegativeGenerator(base.TestCase, BaseNegativeGenerator):
             schema_under_test = copy.copy(valid_schema)
             expected_result = \
                 self.generator.generate_payload(test, schema_under_test)
-            self.assertEqual(expected_result, None)
+            self.assertIsNone(expected_result)
             self._validate_result(valid_schema, schema_under_test)