Added the option to utilise Ganglia on an EMR. 

Allows the user to install [Ganglia](http://ganglia.sourceforge.net/), by using either [Amazon's pre-built bootstrap action](http://docs.aws.amazon.com/ElasticMapReduce/latest/DeveloperGuide/UsingEMR_Ganglia.html) or by allowing the user to specify a specific install script.