Use Celery instead of RQ.

Celery provides a more robust set of queueing options for both tasks and
worker processes. Updates include:
- infrastructure necessary to run Celery, including celery entrypoint
- backgrounded functions are now imported directly from atst.jobs
- update tests as-needed
- update kubernetes worker pod command

celery_worker.py

@@ -0,0 +1,7 @@
+#!/usr/bin/env python
+
+from atst.app import celery, make_app, make_config
+
+config = make_config()
+app = make_app(config)
+app.app_context().push()