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
Why not more directly

```python
devices = [d.split() for d in device_lines if not d.startswith('dm-')]
utils = [(d[0], d[-1]) for d in devices if d]
```