Add MegaCLI collector

This collector exports the following metrics:

- raid_drive_temperature: drive temperature
- raid_drive_count: drive error and event counters
- raid_adapter_disk_presence: disk presence per adapter

node_exporter.conf

@@ -1,7 +1,10 @@
 {
    "attributes" : {
+      "default" : "1",
       "web_server" : "1",
-      "zone" : "a",
-      "default" : "1"
+      "zone" : "a"
+   },
+   "config" : {
+      "megacli_command" : "megacli.sh"
    }
 }