Add support for non-amqp celery brokers OPS-2438

CHANGELOG.md

 - Role: edxapp
+  - Added `EDXAPP_CELERY_BROKER_TRANSPORT` and renamed `EDXAPP_RABBIT_HOSTNAME`
+    to `EDXAPP_CELERY_BROKER_HOSTNAME`. This is to support non-amqp brokers,
+    specifically redis. If `EDXAPP_CELERY_BROKER_HOSTNAME` is unset it will use
+    the value of `EDXAPP_RABBIT_HOSTNAME`, however it is recommended to update
+    your configuration to set `EDXAPP_CELERY_BROKER_TRANSPORT` explicitly.
+
+- Role: edxapp
   - Added `EDXAPP_LMS_SPLIT_DOC_STORE_READ_PREFERENCE` with a default value of
     SECONDARY_PREFERED to distribute read workload across the replica set.
   - Changed `EDXAPP_MONGO_HOSTS` to be a comma seperated string, which is

playbooks/roles/edxapp/defaults/main.yml

@@ -152,6 +152,8 @@ EDXAPP_ZENDESK_CUSTOM_FIELDS: {}
 EDXAPP_ZENDESK_OAUTH_ACCESS_TOKEN: ""
 EDXAPP_CELERY_USER: 'celery'
 EDXAPP_CELERY_PASSWORD: 'celery'
+EDXAPP_CELERY_BROKER_HOSTNAME: "{{ EDXAPP_RABBIT_HOSTNAME }}"
+EDXAPP_CELERY_BROKER_TRANSPORT: 'amqp'
 EDXAPP_CELERY_BROKER_VHOST: ""
 EDXAPP_CELERY_BROKER_USE_SSL: false
 EDXAPP_CELERY_EVENT_QUEUE_TTL: "None"
@@ -1094,8 +1096,8 @@ generic_env_config:  &edxapp_generic_env
       LOCATION: "{{ EDXAPP_CACHE_COURSE_STRUCTURE_MEMCACHE }}"
       # Default to two hours
       TIMEOUT: "7200"
-  CELERY_BROKER_TRANSPORT: 'amqp'
-  CELERY_BROKER_HOSTNAME: "{{ EDXAPP_RABBIT_HOSTNAME }}"
+  CELERY_BROKER_TRANSPORT: "{{ EDXAPP_CELERY_BROKER_TRANSPORT }}"
+  CELERY_BROKER_HOSTNAME: "{{ EDXAPP_CELERY_BROKER_HOSTNAME }}"
   COMMENTS_SERVICE_URL: "{{ EDXAPP_COMMENTS_SERVICE_URL }}"
   LOGGING_ENV: "{{ EDXAPP_LOGGING_ENV }}"
   SESSION_COOKIE_DOMAIN:  "{{ EDXAPP_SESSION_COOKIE_DOMAIN }}"