Task logging levels should be configurable

Related to the nhlib-integration branch: https://github.com/gem/oq-engine/tree/nhlib-integration

We have a decorator called '@oqtask' which is responsible for--among other things--initializing AMQP loggers to be used by celery task code.

At the moment, the log level is hard-coded to 'debug'. This should really be set to the `--log-level` specified when the job is run. This means we should probably store the log level in the oq_job table. (Since the oq_job represents the runtime information about the calculation attempt, I think this is a good place for it. Additionally, the @oqtask decorator is already querying the oq_job from the database to check the status.)