workflows: robotupload task improvement

* Improves the new robotupload task to accept additional
  parameters such as callback_url, nonce for use with BibUpload.

* Adds a new URL route /callback/workflow/continue to receive a callback
  structure to continue a workflow.

* Adds tests for robotupload utils.

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