Have a "stateVersion" setting for instance AMIs

Terraform will do destructive updates when the ami differs. We should probably allow for individuals to pin the default ami chosen so that updates to the amis don't cause unnecessary replacement of cloud resources.

E.g:
```
cluster = {
defaultAMIRelease = "21.05";
....

# for instances
ami = (import (nixpkgs.path + /nixos/modules/virtualisation/ec2-amis.nix)."${cluster.defaultAMIRelease}"."${instance.region".hvm-ebs;
```