Enables "alternatives" and "requires" validator by default for ModelNodeFormBuilder

Currently the "alternatives" and "requires" metadata (from r-r-d) must be explicitly enabled in ModelNodeFormBuilder for the form to take care of those validations, this is to support the same user experience for the forms already in place.
However, as the server-side will validates those metadata anyway, it is better for the UI to take care first and validate it, that said, the validator will be enabled by default.