Fixes privatestorageops#358.

This changes the Throughput graph to be relative, changes its unit
to Bit/s, and fixes the labels of the Network pkt/s graph.

Fixes privatestorageops#24.

Only dashboards in this dir

Fixes privatestorageops#349.

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