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
1 file changed