Less (SLOC) is more

... so Grafana shows us names instead of VPN IP addresses.

Avoiding catastrophy: It's all about minimizing cognitive overhead.

- Add node exporter to nodes
- Make the old nixpkgs 19.09 node exporter work (amend config)
- Fix a typo (??) and exclude monitoringvpn in network traffic graphs to prevent double counting

  sed -i 's|var/secrets|run/keys|'

See Ops#193

... leave some coding fun for later!