Change assertTrue(isinstance()) by optimal assert

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: Ie98c1aec65281fa1784070ce76e472caca7cfc47
Closes-bug: #1268480
diff --git a/tempest/tests/common/test_preprov_creds.py b/tempest/tests/common/test_preprov_creds.py
index 7188e5f..7af8654 100644
--- a/tempest/tests/common/test_preprov_creds.py
+++ b/tempest/tests/common/test_preprov_creds.py
@@ -325,7 +325,7 @@
                                                     'id': 'fake-id',
                                                     'label': 'network-2'}]}):
             creds = test_accounts_class.get_creds_by_roles(['role-7'])
-        self.assertTrue(isinstance(creds, cred_provider.TestResources))
+        self.assertIsInstance(creds, cred_provider.TestResources)
         network = creds.network
         self.assertIsNotNone(network)
         self.assertIn('name', network)