Manually set Cloud-init `instance_id` doesn't work

There's a known issue ( https://bugzilla.proxmox.com/show_bug.cgi?id=1739 ) where "each time you update the cloud-init configuration, or manualy regenerate the image, the instance-id is changed." To ensure that `instance_id` never changes again unless we explicitly do so—the only 'foolproof' solution—we have chosen to write meta-configs with a self-generated `instance_id`. This advice is also…