openstack-image-zfs: drop ".root" suffix in generated image name

hydra-build-products is automatically updated with the right name, as
is image-info.json.

This breaks hardcoded uses of the filename, but ensures that all the
other outputs are consistent and image.filePath is correct

(cherry picked from commit 5aba7c7131)

nixos/maintainers/scripts/openstack/openstack-image-zfs.nix

@@ -113,7 +113,7 @@ in
       postVM = ''
          extension=''${rootDiskImage##*.}
          friendlyName=$out/${config.image.baseName}
-         rootDisk="$friendlyName.root.$extension"
+         rootDisk="$friendlyName.$extension"
          mv "$rootDiskImage" "$rootDisk"
 
          mkdir -p $out/nix-support