"Devices" can be used to specify storage devices on which stats
should be reported. "SkipSerialNumber" can be used to omit
the device serial number.
Added tests to verify the new parameters.
closes#344
Fixed a bug in restricting Disk reporting to specific mountpoints
Added tests for the Disk.Mountpoints option
Fixed minor bug in usage of assert for the cpu tests where expected and actual values were swapped.
Influxdb 0.9.1 now rejects writes with empty tag values. This patch skips tags with no values. A disk serial number does not exist for many devices including md raid arrays and VMs. Other plugins may also want to check for empty tags.