tf-stage1/templates: default keep_disk to true

Gives the option to downgrade a server in the future, similar to the
default on Hetzner's Cloud Console ("CPU and RAM only").

tf-stage1/templates.tf

@@ -110,6 +110,7 @@ resource "hcloud_server" "machine" {
   name        = each.key
   image       = data.hcloud_image.archlinux.id
   server_type = each.value.server_type
+  keep_disk   = true
   location    = "fsn1"
   lifecycle {
     ignore_changes = [image, location]