Add a preflight check to test celery workers (and monitor?) health

[Anthony Lenton]

It would be great to have a preflight check that can test the health of our celery workers, and the celery monitor if possible.

One option would be to provide a "succeed" task that just completes immediately.
The preflight check can then block on that, so that it hits the worker and returns.

As for checking the monitor's health, it might make sense to see if a TaskState object is created, but I'm not sure if this can be made synchronous, or if it will always be slightly delayed.