We want to temporary disable reboot on update for a CoreOS machine, since we haven't implemented high availability of our services yet. Is there away to achieve that other than editing the cloud-init configuration (provided when the machine being provisioned the first time)
CoreOS uses a tool called locksmith to coordinate reboots across the cluster. You can use this tool to temporarily prevent reboots by jumping onto a machine and running
locksmithctl lock, which will indicate to the cluster that a machine has already taken the reboot slot and will keep it occupied until you run
Your best bet however, is to edit the cloud-config.