ganglia: diskstat.py plugin

The diskstat.py Ganglia plugin would generate metrics according to the
content of /proc/diskstats . The files are placed where the recently
introduced ganglia::python::plugin define expect them.

The plugin reads /proc/partitions and strip out any partition containing
a number, in most case that would be what is expected. One can override
that behavior by passing a list of devices as the 'devices' option.

bug: 36994
Change-Id: I2fbb4080c77817f3928c613064de37f3d8863b55