Node-RED's one broker node pointed at zurag.be:1884, where nothing listens. nodered now requires
mqtt-topic (asking for every topic: flows follow the devices' own) and a run-once `mqtt` step —
declared last, restarted when the binding, credential or settings change — points the mesh's broker
nodes at the bound broker through Node-RED's admin API with the module's api-token: the node the
step makes itself when none is named, or the ones an assignment names in `mqtt.brokers`. Only host,
port, TLS and the login change; the broker is asked first whether it takes the login; the deploy is
against the revision read ("nodes", so only that node restarts) and a digest makes a rerun a no-op.
A broker node nobody named is never touched. settings.js keeps `mqtt` and `topics` out of Node-RED.
jschoubben