Ensure Celery tasks get routed to matching worker
Ensure Celery tasks get routed to matching worker

Configure Celery to only send tasks to workers that match the specified
stack color. This prevents tasks from getting assigned to an old worker
in a blue/green deployment.

Configuration is largely based on
http://docs.celeryproject.org/en/latest/userguide/routing.html#manual-routing
and https://github.com/rduplain/celery-simple-example.

Refs #2374

Refs #2374

WikiWatershed/model-my-watershed,WikiWatershed/model-my-watershed,WikiWatershed/model-my-watershed,WikiWatershed/model-my-watershed,WikiWatershed/model-my-watershed