Fix #4726 Update puppet rrdtool metric code to support modern rrd ruby bindings

The rrd project has been shipping ruby bindings since 1.3.0, and the old
rrdtool ruby library is no longer being maintained.

This patch is based upon Davor Ocelic's submitted code, with the
addition that I've added conditionals so it can still call the old
rrdtool library if a modern rrd is not installed.