Move Octavia tests to scenario tests

The Octavia test is similar to the other scenario tests and is currently
the only 'functional' test to use external templates. Make it a scenario
test for consistency, and also so that we can remove the
_load_template() method from the HeatIntegrationTest base class.

Change-Id: Ifaa5b45c96e6e8125b8ba36a349a07b0355b6077
Story: #1739447
Task: 22221
3 files changed