Include queue name in Celery task logs
Include queue name in Celery task logs

This is mainly so we can use it in the new metrics we send to StatsD
in the following commits, but it should also be useful in the logs.
I've taken the opportunity to make the log format consistent between
success / failure, and with our Template Preview app [1].

[1]: https://github.com/alphagov/notifications-template-preview/blob/f456433a5ae7b41661ee014b6589bb31cc2e2c66/app/celery/celery.py#L19

alphagov/notifications-api,alphagov/notifications-api