Make SSH key provisioning explicit and interactive for cloud-init VMs. Default CLOUDINIT_SSH_KEYS is now empty; new helper functions discover and extract public keys from common host files, count them, and present a whiptail menu (import all host keys, paste one key, specify a file, or none). configure_cloudinit_ssh_keys writes selected keys to a temp file and sets CLOUDINIT_SSH_KEYS accordingly (removing the temp file if empty). setup_cloud_init now only applies --sshkeys when CLOUDINIT_SSH_KEYS is explicitly provided and logs the source, and vm/docker-vm.sh invokes the key selection UI for cloud-init VMs.