Only instantiate one celery app per process.

Ticket: BOM-2086

Currently there are parts of the LMS that import content from the CMS
APP and vice-versa.  When this happens, we end up with 2 instances of
the celery app and some tasks get registered to the wrong one. The
tasks that were getting registered to the wrong one are never able to
run and result in lots of production errors on celery workers.

The timing of the CMS celery app instantiation is non deterministic
so different tasks get lost depending on when it's imported by some
code in the LMS.

As long as SERVICE_VARIANT is set, this code should prevent the
instantiation of both celery apps.

cms/__init__.py

@@ -5,7 +5,7 @@
 Import sorting is intentionally disabled in this module.
 isort:skip_file
 """
-
+import os
 
 # We monkey patch Kombu's entrypoints listing because scanning through this
 # accounts for the majority of LMS/Studio startup time for tests, and we don't
@@ -20,4 +20,5 @@
 
 # This will make sure the app is always imported when
 # Django starts so that shared_task will use this app.
-from .celery import APP as CELERY_APP
+if os.environ.get('SERVICE_VARIANT', 'cms').startswith('cms'):
+    from .celery import APP as CELERY_APP

lms/__init__.py

@@ -2,6 +2,7 @@
 Celery needs to be loaded when the cms modules are so that task
 registration and discovery can work correctly.
 """
+import os
 
 # We monkey patch Kombu's entrypoints listing because scanning through this
 # accounts for the majority of LMS/Studio startup time for tests, and we don't
@@ -16,4 +17,5 @@
 
 # This will make sure the app is always imported when
 # Django starts so that shared_task will use this app.
-from .celery import APP as CELERY_APP
+if os.environ.get('SERVICE_VARIANT', 'lms').startswith('lms'):
+    from .celery import APP as CELERY_APP