Optimize the scenario runner

1. Create openstack clients in the ScenarioRunner.run method, i.e. one time
   for all scenario loops (speedup);
2. Get rid of the useless Scenario.class_init() method
3. Move cleanup inside the scenario loop run to enable the cleanup
   implementations to delete the user's resources the scenario was run with;

Blueprint test-engine

Change-Id: I7f8d439baaf4a86539e6cebe7ac83d8fddcd4a47