Update Celery configuration to keep using RabbitMQ as the backend
This updates instance settings so that instances continue to work correctly using RabbitMQ as the Celery backend. This change is required since https://github.com/edx/configuration/pull/6147 switched to local Redis for native install.
We will switch to Redis in Ocim when we implement support for a Redis backend with ACLs, but for now this makes it possible to keep using RabbitMQ.
Test instructions:
I spawned two servers on Ocim stage:
- Juniper: https://stage.manage.opencraft.com/instance/6540/
- master: https://stage.manage.opencraft.com/instance/6530/
Check the instances to make sure they work correctly and the configuration looks good. You can use the default staff@example.com account.
Note: Celery tasks don't seem to be working on the master sandbox, but I think that's related to the problems on edx-platform master where tasks don't get registered properly (see https://github.com/edx/edx-platform/pull/25479#issuecomment-736073800).
Reviewers: