Hi, I'm trying to get LocalStack set up for integration testing and having a lot of trouble building CloudFormation templates that include nested stacks referenced via the `TemplateURL` property. Is this just not a thing that works in LocalStack? I've tried setting TemplateURL to a local file as well as to an object in an S3 bucket created with LocalStack.

For example if I call `create-stack` on a `template.yaml` containing the following:
```
Resources:
  SNSTopics:
    Type: AWS::CloudFormation::Stack
    Properties:
      TemplateURL: http://localhost:4572/artifacts/sns.yaml
```

...the stack builds but the referenced `sns.yaml` stack does not get built; whereas if I call `create-stack` directly on the referenced file it builds fine.

Thanks for any help/advice you can give!
props = cloudformation_json['Properties']
        name = props['Name']
        region_name = props.get('Region') or aws_stack.get_region()
        description = props.get('Description') or ''
        id = props.get('Id') or short_uid()
        return apigw_models.RestAPI(id, region_name, name, description)
    def RestAPI_get_cfn_attribute(self, attribute_name):
        if attribute_name == 'Id':
            return self.id
        if attribute_name == 'Region':
            return self.region_name
        if attribute_name == 'Name':
            return self.name
        if attribute_name == 'Description':
            return self.description
        if attribute_name == 'RootResourceId':
            for id, resource in self.resources.items():
                if resource.parent_id is None:
                    return resource.id
            return None
        raise UnformattedGetAttTemplateException()
    @classmethod
    def Deployment_create_from_cloudformation_json(cls, resource_name, cloudformation_json, region_name):