Simplification of changes introduced in 19c7c6d95e8a3618d8e779f4c2c9fec6cbe5348a to avoid modifying config vars

i think these changes to 19c7c6d95e8a3618d8e779f4c2c9fec6cbe5348a may make it easier when using a custom ami as you don't need to edit things in scripts outside the cluster.yml