workflows: abstraction layer for workers
- Adds an abstract class to be used like a base for worker wrapper.
- Implements a worker wrapper for abstraction for celery worker.
- Starts the implementation of a worker wrapper for redis worker.
- Fixes PEP8 and sphinx documentation specification.
- Fixes mistake about iteritems with six.
Signed-off-by: Guillaume Lastecoueres <guillaume.lastecoueres@cern.ch>
Reviewed-by: Jan Aage Lavik <jan.age.lavik@cern.ch>