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: