Allow setting custom configurations in `elasticsearch.yml`

Added a block of code which will print all key:value pairs set in the
`node.elasticsearch[:custom_config]` attribute in the `elasticsearch.yml` file:

See Vagrantfile for example.

Fixes: #59, Related: #64
Sort iteration of logging config in `logging.yml` to prevent needless restarts

In practice, <% node.elasticsearch[:logging].each do |key, value| %> seems to have a non-deterministic order in Ruby < 1.9,
thus Chef will trigger a restart of the ElasticSearch server every time the chef-client is run.

Closes #64