Renaming 'on-finish' to 'on-complete' in task spec

* Renamed 'on-finish' to 'on-complete'
* Added unit tests
* Renamed obsolete LinearWorkflowHandlerTest to DirectWorkflowHandlerTest

Change-Id: Ia88a1f6c0ebc4cd81cb1de7cd8b8deb18d105251
Merge "Renaming 'on-finish' to 'on-complete' in task spec"