Proxy ElasticSearch with mod_rewrite.

* modules/logstash/templates/kibana.vhost.erb: Use mod_rewrite to proxy
elasticsearch so that we can safely proxy _mapping. To do this safely we
need to restrict requests to GETs. Also, add _nodes to the list of
proxies URIs.

Change-Id: Ibe86dc104c429c144b31fed547034ee14f7f2e1d
Merge "Proxy ElasticSearch with mod_rewrite."