Monitor MegaRAID on our storage nodes

Add a MegaCli to Prometheus Text format Exporter for the Storage nodes.

• Add MegaCli
• Add megacli2prom.py script
• Add systemd service (or crontab or so) to run that every five minutes or so
• Add a dashboard and alerts in Grafana

Fixes privatestorageops#19.

So far only a super simple dashboard, no drive errors or so in lieu of experience here, just the number degraded arrays and an alert if that is greater than 0:

nixos/modules/monitoring/exporters/node.nix

@@ -35,8 +35,8 @@ in {
       #"softnet" # not in nixpkgs 19.09
       "stat"
       "systemd"
-      # "textfile"
-      # "textfile.directory /run/prometheus-node-exporter"
+      "textfile"
+      "textfile.directory /run/prometheus-node-exporter"
       #"thermal_zone" # not in nixpkgs 19.09
       "time"
       #"udp_queues" # not in nixpkgs 19.09