Put its contents in it and then use them from there instead of importing them
all over the rest of the code.

Also change the `domain` parameter for Grafana to always
use that beautiful name - besides display it's cosmetic anyway
IIANM so I'll not parametrize it for now.

... clean up 

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