ganglia: diskstats plugin using wrong default

The DEVICES global is supposed to list all devices we want to monitor,
whenever it is not set in diskstat.pyconf, it would end up being set to
None ( {}.get('nonexistent') -> None) but the device lookup code only
triggers when DEVICE is an empty string.

Change-Id: I167393e80d76d076be4d1823ef88aa304407be79