Elasticsearch improvements

* !!IMPORTANT!! The elasticsearch data directory was being set by the transgressive attribute 'data_root'; worse, the scratch space was locked to being a subdirectory of that. Now actually uses the volume_dirs. This is necessary for switching to the 'local' gateway mode.
          - node[:elasticsearch][:data_dir], set by the :elasticsearch_data tag on a volume, defines where data goes ('ES_DATA_DIR');
          - node[:elasticsearch][:scratch_dir], set by the :elasticsearch_scratch tag on a volume, now specifies the scratch dir ('ES_WORK_DIR')
          - if you were using :data_root, it will complain and make you change it. The work_dir may change, but that shouldn't matter to you.
* Hosts in discovery are sorted so their order in the list doesn't dance
* Added the 'head' and 'bigdesk' dashboard plugins. Neato!
* Changed the plugins definition to use a hash, not a string. It will still accept your string, but it will holler. I might make it just break instead, so update your overrides.
* Added an 'overall' log setting -- use it to set the baseline log4j log level
* if you are a data node, node.name is set to the machine name and 'max_local_storage_nodes' is set to one.
* set the minimum_master_nodes value to 1 + (number of seeds / 2) to prevent split-brain in clusters of size > 2