Customize attributes easier

I'm using AWS OpsWorks (Chef 11.10)

I was struggling to wrap this cookbook to change some of the attributes.
This pull request adds in a blank customize.rb attribute file that you can override in your own custom elasticsearch cookbook as specified here: http://docs.aws.amazon.com/opsworks/latest/userguide/workingcookbook-cookbook-attributes.html

My Opsworks repository URL points to a git repo (https://github.com/stuart-warren/opsworks-cookbooks) containing:
* a Berksfile that pulls the actual elasticsearch cookbook
* a minimal custom elasticsearch cookbook, just containing the attributes/customize.rb with my attributes (at 'normal' priority). This is merged with the actual cookbook during the chef run.

I then just run the normal recipies:
elasticsearch elasticsearch::aws elasticsearch::plugins elasticsearch::proxy