Fewer lines almost always better.  Also, we don't want to spread
these out unless we have to.

Seems this is opt-out:
https://github.com/NixOS/nixpkgs/blob/9d1350d9d56411b685ff3de5839ed6728b1bf808/nixos/modules/services/monitoring/grafana.nix#L576-L582

Known issues:

  - Error rate tooltip computes over whole range (sum), should be per second or per minute or so
  - Duration buckets not great. Should also be per second probably.

We have enough numbers with little value already

Make everything more consistent and generally more nice.
I am a bit proud of this now.
Let's hope it holds up.

Rationale: These are servers, they should be transmitting mostly.
But really it's rather arbitrary.

- 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