refactor: add CELERY_RESULT_BACKEND with default django-cache

CHANGELOG.md

@@ -4,6 +4,11 @@
 All notable changes to this project will be documented in this file.
 Add any new changes to the top (right below this line).
 
+ - 2022-06-08
+    - Role: edxapp
+        - Added a new `CELERY_RESULT_BACKEND` setting to allow operators to
+          override the default celery result backend.
+
  - 2022-04-06
     - Role: edxapp
         - Added a new `EDXAPP_COMPILE_JSI18N` variable to control whether

docker/build/edxapp/lms.yml

@@ -112,6 +112,7 @@ CELERY_QUEUES:
 - edx.lms.core.high
 - edx.lms.core.high_mem
 CELERY_TIMEZONE: UTC
+CELERY_RESULT_BACKEND: django-cache
 CERTIFICATE_TEMPLATE_LANGUAGES:
     en: English
     es: Español

docker/build/edxapp/studio.yml

@@ -92,6 +92,7 @@ CELERY_QUEUES:
 - edx.cms.core.default
 - edx.cms.core.high
 CELERY_TIMEZONE: UTC
+CELERY_RESULT_BACKEND: django-cache
 CERTIFICATE_TEMPLATE_LANGUAGES:
     en: English
     es: Español

playbooks/roles/edxapp/defaults/main.yml

@@ -181,6 +181,7 @@ EDXAPP_CELERY_BROKER_USE_SSL: false
 EDXAPP_CELERY_EVENT_QUEUE_TTL: !!null
 EDXAPP_CELERY_TIMEZONE: "UTC"
 EDXAPP_CELERYBEAT_SCHEDULER: "celery.beat:PersistentScheduler"
+EDXAPP_CELERY_RESULT_BACKEND: "django-cache"
 EDXAPP_ENABLE_CELERY_BEAT: false
 EDXAPP_SINGLE_BEAT_LOCK_TIME: 60
 # EDXAPP_SINGLE_BEAT_HEARTBEAT_INTERVAL must be smaller than EDXAPP_SINGLE_BEAT_LOCK_TIME / 2
@@ -1323,6 +1324,7 @@ generic_env_config:  &edxapp_generic_env
   CELERY_BROKER_VHOST: "{{ EDXAPP_CELERY_BROKER_VHOST }}"
   CELERY_BROKER_USE_SSL: "{{ EDXAPP_CELERY_BROKER_USE_SSL }}"
   CELERY_EVENT_QUEUE_TTL: "{{ EDXAPP_CELERY_EVENT_QUEUE_TTL }}"
+  CELERY_RESULT_BACKEND: "{{ EDXAPP_CELERY_RESULT_BACKEND }}"
   PAYMENT_SUPPORT_EMAIL: "{{ EDXAPP_PAYMENT_SUPPORT_EMAIL }}"
   ZENDESK_URL: "{{ EDXAPP_ZENDESK_URL }}"
   ZENDESK_CUSTOM_FIELDS: "{{ EDXAPP_ZENDESK_CUSTOM_FIELDS }}"