Add interactive SSH key selection for cloud-init

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.

vm/docker-vm.sh

@@ -169,6 +169,11 @@ function advanced_settings() {
   select_os
   select_cloud_init
 
+  # SSH Key selection for Cloud-Init VMs
+  if [ "$USE_CLOUD_INIT" = "yes" ]; then
+    configure_cloudinit_ssh_keys || true
+  fi
+
   METHOD="advanced"
   [ -z "${VMID:-}" ] && VMID=$(get_valid_nextid)