Add iostat graphing

[carbontwelve]

• http://www.thattommyhall.com/2011/02/18/iops-linux-iostat/
• http://taoofmac.com/space/cli/RRDTool/Snippets
• http://cpansearch.perl.org/src/NICOLAW/RRD-Simple-1.44/examples/iostat.pl
• https://github.com/sh2/iostat2graphs/blob/master/iostat2graphs.pl
• http://wiki.creatica.org/cgi-bin/wiki.pl/Performance_Monitoring_with_Nagios_and_RRDTool_-_pnp4Nagios

[carbontwelve]

3197ecc0b6f207a9216e83d169acdbe8122f72a3 added the disk utilization graph, not sure if I am collecting the stats from iostat correctly.