Add collectd disk plugin graphing to rrd.py

Added DiskPlugin to rrd.py to enable graphing of the collectd disk read- and write-octects datasource

Related: http://forums.freenas.org/threads/reporting-gui-page-reporting-jails-that-have-been-deleted.14217/#post-89878

I'm not the member on the forum who originally posted the code, but I needed the change also.  I improved the originally posted code by correctly formatting and aligning the legend labels and removing a disk name filter that was there.