Improvement: Add option to drop back to previous taskdef if update fails

[mikestead]

Currently when you launch an update, if the new task fails to come up, or is brought down due to a failing health check, you can be in a situation where ECS service keeps trying to bring back up this failing task even though this deploy tool will have reported the failure.

To avoid this we should add option to rollback the service to the previous task def if the update fails.