Mark config file as sensitive (#67)

This file contains passwords and other secrets, treating it as sensitive
prevents Puppet from showing the diff and thus leaking the password in
reports.

manifests/server.pp

@@ -47,7 +47,7 @@
     owner   => $user,
     group   => $group,
     mode    => '0640',
-    content => template("${module_name}/server/graylog.conf.erb"),
+    content => Sensitive(template("${module_name}/server/graylog.conf.erb")),
   }
 
   case $facts['os']['family'] {