meta model subscriber on resource registration.

        We watch for new resource types being registered and if they
        support aws config, automatically register the jsondiff filter.

def register_resources(klass, registry, resource_class):
        """ meta model subscriber on resource registration.

        We watch for new resource types being registered and if they
        support aws config, automatically register the jsondiff filter.
        """
        config_type = getattr(resource_class.resource_type, 'config_type', None)
        if config_type is None:
            return
        resource_class.filter_registry.register('json-diff', klass)