Option to specify engine output destination

[Lars Butler]

We should be able to tell the engine where calculation results should go (XML files vs. DB). For starters, this could be implemented as a command line option (--output=db or --output=xml).

[Mattia Barbon (mattia.barbon)]

Solution outline:

- allow creating uiapi.oq_job without a corresponding upload
  * make the uiapi.oq_params.upload_id column nullable
  * make the uiapi.oq_job.path column nullable

- add an output_type parameter to bin/openquake
- when loading a job from file with output_type=db, create a new uiapi.oq_job, with the corresponding oq_params
  * for now always use the 'openquake' user, requires integrating with the authentication blueprint
- for each artifact type, add the code to serialize to DB