Task discovery expects a default queue if celery is set up

Preamble

Task discovery can be launched at start-up (--rediscover-tasks) or from a API call (/tasks/discover). It is actually a job submitted to a queue if celery is set-up.

Problem

But for now, there is no way of specifying the queue: task discovery is only submitted to the default queue (called celery).

This means that the default queue needs to exist but also that there is no way to launch task discovery on workers listening to other queues.

Proposal

The task discovery should not be arbitrary be submitted to the default queue. It should go through all available queues and run the discovery there. Unlike job submission, I don't see the point of being able to specify the queue: all queues should be "discovered" at once.