Set ElasticSearch output node name to hostname.

* modules/openstack_project/templates/logstash/indexer.conf.erb: Set the
node name parameter in the ElasticSearch output template to be the
hostname of the host running logstash.

Change-Id: I8ade373403ca59d6d5768df175405167c9c8b4be
Reviewed-on: https://review.openstack.org/30925
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Elizabeth Krumbach Joseph <lyz@princessleia.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins

indexer.conf.erb

@@ -77,7 +77,7 @@ filter {
 output {
   elasticsearch {
     host => "elasticsearch.openstack.org"
-    node_name => "logstash-indexer"
+    node_name => "<%= scope.lookupvar("::hostname") %>"
     max_inflight_requests => 512
   }
 }