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:

morph/lib/storage.nix

@@ -43,8 +43,14 @@ in {
     ../../nixos/modules/monitoring/vpn/client.nix
     # Expose base system metrics over the monitoringvpn.
     ../../nixos/modules/monitoring/exporters/node.nix
+    # At least some of our storage nodes utilize MegaRAID storage controllers.
+    ../../nixos/modules/monitoring/exporters/megacli2prom.nix
   ];
 
+  # Enable statistics gathering for MegaRAID cards.
+  # TODO would be nice to enable only on machines that have such a device.
+  services.private-storage.monitoring.megacli2prom.enable = true;
+
   # Turn on the Private Storage (Tahoe-LAFS) service.
   services.private-storage = {
     # Yep.  Turn it on.