Rearrange celery options logic to make clearer
Rearrange celery options logic to make clearer

Hopefully this makes it clearer at a glance that either the predefined
url or the prefix is used. It was easy before to stop reading after the
initial set of options was defined, and miss it being removed and
replaced

(cherry picked from commit b64c75be499122c3df8037ac014fdd5c9ffadc61)

ministryofjustice/cla_backend,ministryofjustice/cla_backend,ministryofjustice/cla_backend,ministryofjustice/cla_backend