Isolate cluster-dependent Celery startup

Avoids calling unworkable functions when generating API docs etc. by
isolating them into a Celery startup function called by Daemon.py.

Also update to Celery 4+ settings format.

api-daemon/pvcapid/Daemon.py

@@ -155,6 +155,7 @@ def entrypoint():
     print("|----------------------------------------------------------|")
     print("")
 
+    pvc_api.celery_startup()
     pvc_api.app.run(
         config["listen_address"],
         config["listen_port"],