oaiharvester: decorating inner function in tasks

• Wraps workflow tasks to get outer function's attributes.

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