fix:pass celery always_eager flag as it is consumed by celery4
fix:pass celery always_eager flag as it is consumed by celery4

Based on
https://github.com/celery/celery/blob/04d921b3fbcd9cc42285edeb916c41df507fe691/docs/history/whatsnew-4.0.rst
this and several other celery settings changed in the course of our
py3.8 upgrade. This is one which matters a great deal for
sandboxes (which shouldn't rely on celery for the execution of asynch
tasks). There are some more which I'll follow up with after I
understand better why their apparent lack isn't impacting production
builds' operation of celery.

edx/configuration,mitodl/configuration,mitodl/configuration,edx/configuration,arbrandes/edx-configuration,mitodl/configuration,edx/configuration,arbrandes/edx-configuration,arbrandes/edx-configuration,mitodl/configuration,edx/configuration,arbrandes/edx-configuration,arbrandes/edx-configuration