openstackImage: set the / fsType to reenable root FS resizing

Since 34234dcb51, the reisizefs tool is
embeded only if the `fsType` starts with `ext`. The default `fsType`
value is `auto`.

nixos/modules/virtualisation/openstack-config.nix

@@ -20,6 +20,7 @@ in
   config = {
     fileSystems."/" = {
       device = "/dev/disk/by-label/nixos";
+      fsType = "ext4";
       autoResize = true;
     };