Renaming 'stop_workflow' to 'pause_workflow'

* Renamed method 'stop_workflow' to 'pause_workflow' for engine
  and workflow handler.
* Renamed state 'STOPPED' to 'PAUSED' according to the name of
  engine instruction 'pause'

Change-Id: Iee549fe25e93d68a6c15b2ce2c5f2da0df9af080