workflows: aborting and skipping

* Adds support in the workflow engine for aborting execution and
  skipping objects using new functions `eng.skipToken()` and
  `eng.abortProcessing()` that will not save the workflow object.
  Useful in complex workflow where for example the workflow object
  is deleted as a final step in a task before aborting/skipping.

* Removes legacy DEBUG flag from engine as logging is anyway logged
  with the debug loglevel.

Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>
Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>