Use WithScenarios base class instead of load_tests

To support test scenarios, we now override load_tests in the module
using them. To support alternate test runners and not needing that, the
patch makes the Heat test base class inherit testsceanios.WithScenarios.

Change-Id: Ie2435e41c466c0687bb1dd95080dcac7c837aaba
Merge "Use WithScenarios base class instead of load_tests"