Working on reverse workflow: implementing method start_workflow()

* Implementing method start_workflow() in ReverseWorkflowHandler
* Fixing usages of specification v1 due to changes in BaseSpec class

TODO:
* Finish the rest in ReverseWorkflowHandler (on_task_result() etc.)

Change-Id: I5063ff2ad0395e8b77b3c7de6fb41173411139ea