Use assertIn and assertNotIn instead of assertTrue/assertFalse

The assertIn and assertNotIn is defined in testtools, it is expected to
work with all testtools supported python version.

The assertIn and assertNotIn is more verbose when a real issue occurs.

Change-Id: I7a21a6e38f606deb58b5bade6414f6c4a4142f4b
diff --git a/tempest/api/compute/test_extensions.py b/tempest/api/compute/test_extensions.py
index 291c8e4..4359c49 100644
--- a/tempest/api/compute/test_extensions.py
+++ b/tempest/api/compute/test_extensions.py
@@ -27,7 +27,7 @@
     def test_list_extensions(self):
         # List of all extensions
         resp, extensions = self.extensions_client.list_extensions()
-        self.assertTrue("extensions" in extensions)
+        self.assertIn("extensions", extensions)
         self.assertEqual(200, resp.status)