Added Elasticsearch AWS discovery for logstash.

Added a recipe to enable the embedded client in the logstash output
to automatically discover elasticsearch nodes running on Amazon EC2.
The default mulicast discovery will not work on EC2.

  * Download the cloud-aws plugin, unzip it and link it the logstash
  home directory. Uses the ark cookbook.
  * Modified the logstash startup scripts to specify a classpath
  instead of the jar argument. Optionally include the cloud-aws plugin
  on the classpath.
  * Place an elasticsearch.yml config file in the logstash home
  directory. This file contains the cluster name, AWS access
  key, etc.