Add a cluster name option

This is useful if you want to run multiple clusters, or if you just want
to change the default value to something custom.

Change-Id: Id1731a209b43057090852a3bc484f61529717bd3

templates/elasticsearch.yml.erb

@@ -30,6 +30,9 @@
 # multiple clusters on the same network, make sure you're using unique names.
 #
 # cluster.name: elasticsearch
+<% if es_template_config.has_key?('cluster.name') then -%>
+cluster.name: <%= es_template_config['cluster.name'] %>
+<% end -%>
 
 
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