workflows: runtime based start_async_workflow

* Reworks the start_async_workflow() task to allow for a runtime
  based evaluation of which workflow to launch instead of being
  determined on application load.

Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
oaiharvester: sample approval based workflow

* Adds a sample approval based workflow for OAI harvesting with
  the intent of showcasing the Holding Pen approval action and
  the new way of creating OAI harvest workflows.

Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>