replace assertTrue(isinstance) to assertIsInstance

some of tests use different method of assertTrue(isinstance(A, B))
or assertEqual(type(A), B). The correct way is to use
assertIsInstance(A, B) provided by testtools

Change-Id: Idd931d27b13a46800e27393a98da096d41442320
Closes-bug: #1268480

ceilometerclient/tests/v2/test_alarms.py

@@ -323,7 +323,7 @@ class AlarmManagerTest(testtools.TestCase):
         self.assertEqual(self.api.calls, expect)
         for i in xrange(len(history)):
             change = history[i]
-            self.assertTrue(isinstance(change, alarms.AlarmChange))
+            self.assertIsInstance(change, alarms.AlarmChange)
             for k, v in six.iteritems(ALARM_HISTORY[i]):
                 self.assertEqual(getattr(change, k), v)