[Maas-devel] We might want to upgrade to celery 3.0

[Julian Edwards]

It came out in June last year but seems not to be packaged as the celery
in Saucy is still on the same version that was distributed with Quantal.

There's a few nice changes:

- new api (old one still available)
- chaining of tasks now possible, also with callbacks on groups of tasks
- uses librabbitmq C lib instead of py-amqp (one fewer Python dependency)
- threadless; performs better
- Now uses a single "celery" command line which wraps up some
interesting management commands.
- runs on Python 3

Also while I am talking about celery, I noticed that it has fanout
queues called "Broadcast" queues.  This will enable us to easily solve
the region worker SPoF problem.

Sadly there doesn't seem to be anything built-in to detect missing
workers, so we'll probably have to DIY that.