elasticsearch: move config files into etc.

To persist them over upgrade.

Closes Homebrew/homebrew#37037.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Formula/elasticsearch.rb

@@ -68,6 +68,10 @@ class Elasticsearch < Formula
       # Replace paths to use libexec instead of lib
       s.gsub!(%r{\$ES_HOME/lib/}, "$ES_CLASSPATH/")
     end
+
+    # Move config files into etc
+    (etc/"elasticsearch").install Dir[prefix/"config/*"]
+    (prefix/"config").rmtree
   end
 
   def post_install
@@ -75,12 +79,14 @@ class Elasticsearch < Formula
     (var/"elasticsearch/#{cluster_name}").mkpath
     (var/"log/elasticsearch").mkpath
     (var/"lib/elasticsearch/plugins").mkpath
+    ln_s etc/"elasticsearch", prefix/"config"
   end
 
   def caveats; <<-EOS.undent
     Data:    #{var}/elasticsearch/#{cluster_name}/
     Logs:    #{var}/log/elasticsearch/#{cluster_name}.log
     Plugins: #{var}/lib/elasticsearch/plugins/
+    Config:  #{etc}/elasticsearch/
     EOS
   end