oaiharvester: update model with defaults

* Adds better defaults for the oaiharvester model.
* Changes the argument column to MarshalBinary and
  removes getter get_arguments().

Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
workflows: update calls to OaiHarvest model

* Updates the calls to get the arguments dict from the
  OaiHarvest model to adhere to recent API changes.

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