Refactoring local and scalable engines

* Moved iethods start_workflow_execution and convey_task_result into
 abstract engine
* Refactored existing local engine unit tests
* Added simple unit tests for scalable engine
* Minor cosmetic changes (imports, blank lines)

Change-Id: Ic781b134068f52ed5827b945fb57b202239fe9c4
Merge "Refactoring local and scalable engines"