Luci builder configs have now been available in separate repos. This CL enables validation of the config json file format for all engine recipes. Change-Id: Ib60e69a4e920c4199b03323b10bc8c30f708b211 Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/5300 Reviewed-by: Tong Wu <wutong@google.com> Commit-Queue: Keyong Han <keyonghan@google.com>