update_in_cache on lms worker (#12689)

This commit "undoes"a previous hotfix, and allows a cms course_publish
signal to trigger a block_structure update_course_in_cache task, which
is run on an lms worker queue.

Changes:
    -exposes ALTERNATE_QUEUE_ENVS
    -adds routing layer in celery.py
    -moves prior dev_with_worker settings file to devstack_with_worker
    -moves course_block api functionality into openedx/core/djangoapps/content/block_structure

cms/envs/aws.py

@@ -87,6 +87,21 @@ CELERY_QUEUES = {
     DEFAULT_PRIORITY_QUEUE: {}
 }
 
+# Setup alternate queues, to allow access to cross-process workers
+ALTERNATE_QUEUE_ENVS = os.environ.get('ALTERNATE_WORKER_QUEUES', '').split()
+ALTERNATE_QUEUES = [
+    DEFAULT_PRIORITY_QUEUE.replace(QUEUE_VARIANT, alternate + '.')
+    for alternate in ALTERNATE_QUEUE_ENVS
+]
+CELERY_QUEUES.update(
+    {
+        alternate: {}
+        for alternate in ALTERNATE_QUEUES
+        if alternate not in CELERY_QUEUES.keys()
+    }
+)
+CELERY_ROUTES = "{}celery.Router".format(QUEUE_VARIANT)
+
 ############# NON-SECURE ENV CONFIG ##############################
 # Things like server locations, ports, etc.
 with open(CONFIG_ROOT / CONFIG_PREFIX + "env.json") as env_file:

cms/envs/devstack_with_worker.py

@@ -1,32 +1,24 @@
 """
-This config file follows the dev enviroment, but adds the
+This config file follows the devstack enviroment, but adds the
 requirement of a celery worker running in the background to process
 celery tasks.
 
-The worker can be executed using:
+When testing locally, run lms/cms with this settings file as well, to test queueing
+of tasks onto the appropriate workers.
 
-django_admin.py celery worker
+In two separate processes on devstack:
+    paver devstack studio --settings=devstack_with_worker
+    ./manage.py cms celery worker --settings=devstack_with_worker
 """
 
 # We intentionally define lots of variables that aren't used, and
 # want to import all variables from base settings files
 # pylint: disable=wildcard-import, unused-wildcard-import
+from cms.envs.devstack import *
 
-from dev import *
-
-################################# CELERY ######################################
-
-# Requires a separate celery worker
-
+# Require a separate celery worker
 CELERY_ALWAYS_EAGER = False
 
-# Use django db as the broker and result store
-
-BROKER_URL = 'django://'
-INSTALLED_APPS += ('djcelery.transport', )
-CELERY_RESULT_BACKEND = 'database'
-DJKOMBU_POLLING_INTERVAL = 1.0
-
 # Disable transaction management because we are using a worker. Views
 # that request a task and wait for the result will deadlock otherwise.
 for database_name in DATABASES: