Revise db users and django/ORM router logic

[Muharem Hrnjadovic]

We need to re-structure the database users to retain some degree of
control but still make it easy to use the django ORM.

The openquake process roughly consists of the following phases:

    1 - job initialization
        - write NRML files to hzrdi.* and riski.* tables, insert the
          uiapi.input, uiapi.oq_params and uiapi.oq_job rows
        - spawn the job supervisor and update the uiapi.oq_job record
          with the supervisor's pid.
        [suggested db user: oq_job_init]
    2 - calculation
    3 - result serialization: write hzrdr.* and riskr.* tables, insert
        an uiapi.output record.
        [suggested db user: oq_reslt_writer]

On the job supervisor side we have the following actions:

    1 - update the uiapi.oq_job record with the job's status
    2 - potentially insert rows into uiapi.error_msg
    [suggested db user: oq_job_supervisor]

In light of the above the only point of ambiguity for the django/ORM
router code is the write access to uiapi.oq_job. It can either be
requested by oq_job_init or by oq_job_supervisor.

The resolution is to utilise something like:

    OqJob.objects.using('job_supervisor').filter()

and

    job.save(using='job_supervisor')

in the job supervisor code.

We need to add the following db users:

    oq_job_init
    oq_job_supervisor

We also need to delete the following users:

    oq_hzrdi_reader
    oq_hzrdi_writer
    oq_reslt_reader
    oq_riski_reader
    oq_riski_writer
    oq_uiapi_reader
    oq_uiapi_writer