rough sketch of using a tag to make creating instances idempotent

This is something I'm using at the mo which is great for shortening feedback loop for building up infrastructure. It uses a role tag to define how many instances with a certain role should be running. That way if 5/6 roles are working fine, you can simply kill the boxes in the 6th broken role, rerun your playbook, and only those instances are recreated.

Let me know if this seems a desirable goal. Also - perhaps the tag used for defining a role for a group of servers should be parameterised?