Disk utilisation plugin.

Returns metrics for all devices listed by iostat that do not start with
'dm-'. The metric is the %util over a period of 5 seconds from when the
check is run.

Required by the CDM checks/alarms patch in ansible-lxc-rpc
If there's a device line that once split is an empty list, do we really want to return `status_err`? I would think an `IndexError` wouldn't be cause for erroring out, rather we should make sure we don't hit that by checking for empty lists above.