Fix idempotent id for heat scenario tests

Heat scenario test define a Tempest idempotent id, however since
the base class does not inherit from WithAttributes, the id is not
added to the test function, and not displayed when listing or
invoking the id() method.

Fixing that by adding inheritance to testtools.testcase.WithAttributes.

Change-Id: Id3afee4d36a13967f9da11fc1f4c1a0576f0c3fd
diff --git a/heat_tempest_plugin/common/test.py b/heat_tempest_plugin/common/test.py
index 4fdfb8e..dbd2e27 100644
--- a/heat_tempest_plugin/common/test.py
+++ b/heat_tempest_plugin/common/test.py
@@ -86,7 +86,8 @@
     return skipper(test_method)
 
 
-class HeatIntegrationTest(testscenarios.WithScenarios,
+class HeatIntegrationTest(testtools.testcase.WithAttributes,
+                          testscenarios.WithScenarios,
                           testtools.TestCase):
 
     def setUp(self):