Elasticsearch

NOTE: Clustering is done automatically by elasticsearch (using zen discovery) as it is not possible to reconfigure zen discovery at runtime. As a future improvement, cross-cloud support could be added using tribes
See my [blog post](http://abstractvisitorpattern.blogspot.com/2014/03/brooklyn-configkey-and-entityspec.html) on the subject ;) You can also miss off the generic parameter and say `EntitySpec<?>` to get rid of the pointless cast, I think.