fix: revert Celery command change for services using Celery<5

[Agrendalath]

Fixes #186. Celery was upgraded only for the edx-platform. Other repositories are still using Celery<5.

This directly affects ecommerce-worker and breaks the deployment process when SANDBOX_ENABLE_ECOMMERCE: true is defined.

The /edx/var/log/supervisor/ecomworker-stderr.log file contains:

Error: 
Unable to load celery application.
Module 'ecommerce_worker' has no attribute 'celery'

The following services could also be affected:

• registrar
• discovery
• enterprise_catalog
• license_manager

However, I've noticed something odd in the edx_django_service playbook. While supervisor worker configs are created for these services, no task would enable them. There is only a task which creates and enables the config for the service's main process.

FYI: @gabor-boros

[gabor-boros]

@Agrendalath Nice! Could you please cherry-pick this to maple as well?

[Agrendalath]

@gabor-boros, we don't have the Maple branch ready yet, so I've left a note on BB-6123.