Make Elasticsearch Kibana config an array by default

This will make it easier to supply multiple ES nodes
in wrapper cookbooks. You simply need to specify a list
of ES nodes in node.kibana.elasticsearch.hosts attribute.
Kibana is smart enough to recognize and handle this properly.
Modified the README file to describe the new attribute change
Merge pull request #6 from milosgajdos83/elasticsearch-config

Elasticsearch Kibana config