Yes, having predefined keys in your VM images does the trick, and is exactly what we do for (almost) zero-intervention deployments of our servers in my particular environment.
We tend to destroy and recreate servers more often than we scale out, so we haven't bothered to remove the manual step of adding the server's hostname to the ansible inventory_hosts file. However, that's easily automatable...
Ansible will _execute_ your inventory_hosts file if it's executable, and IIRC it just needs to return a JSON or YML data structure representing all your servers and the groups they're in. So, as long as you have a library which can query your infrastructure (e.g. boto for EC2 etc) it's not hard to automate this.
We tend to destroy and recreate servers more often than we scale out, so we haven't bothered to remove the manual step of adding the server's hostname to the ansible inventory_hosts file. However, that's easily automatable...
Ansible will _execute_ your inventory_hosts file if it's executable, and IIRC it just needs to return a JSON or YML data structure representing all your servers and the groups they're in. So, as long as you have a library which can query your infrastructure (e.g. boto for EC2 etc) it's not hard to automate this.