get rid of celery/celerybeat

alternative to https://github.com/ansible/awx/pull/2530 which makes use
of https://pypi.org/project/schedule/

this doesn't have support for any persistence (like how celery beat uses
a shelve file), because all of our periodic jobs run at most every few
minutes

awx/main/dispatch/periodic.py

@@ -0,0 +1,52 @@
+import logging
+import threading
+import time
+
+from django.conf import settings
+from django.db import connections
+from schedule import Scheduler
+
+from awx.main.dispatch.worker import TaskWorker
+
+logger = logging.getLogger('awx.main.dispatch.periodic')
+
+
+class Scheduler(Scheduler):
+
+    def run_continuously(self):
+        cease_continuous_run = threading.Event()
+        idle_seconds = max(
+            1,
+            min(self.jobs).period.total_seconds() / 2
+        )
+
+        class ScheduleThread(threading.Thread):
+            @classmethod
+            def run(cls):
+                while not cease_continuous_run.is_set():
+                    try:
+                        for conn in connections.all():
+                            # If the database connection has a hiccup, re-establish a new
+                            # connection
+                            conn.close_if_unusable_or_obsolete()
+                        self.run_pending()
+                    except Exception:
+                        logger.exception(
+                            'encountered an error while scheduling periodic tasks'
+                        )
+                    time.sleep(idle_seconds)
+                logger.debug('periodic thread exiting...')
+
+        thread = ScheduleThread()
+        thread.daemon = True
+        thread.start()
+        return cease_continuous_run
+
+
+def run_continuously():
+    scheduler = Scheduler()
+    for task in settings.CELERYBEAT_SCHEDULE.values():
+        apply_async = TaskWorker.resolve_callable(task['task']).apply_async
+        total_seconds = task['schedule'].total_seconds()
+        scheduler.every(total_seconds).seconds.do(apply_async)
+    return scheduler.run_continuously()