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!
original_resource, new_resource_name, cloudformation_json, region_name):
        resource_name = cloudformation_json.get('Properties', {}).get('FunctionName') or new_resource_name
        return Lambda_create_from_cloudformation_json_orig(resource_name, cloudformation_json, region_name)
    if not hasattr(lambda_models.LambdaFunction, 'update_from_cloudformation_json'):
        lambda_models.LambdaFunction.update_from_cloudformation_json = Lambda_update_from_cloudformation_json
    
    def depl_delete_from_cloudformation_json(
            resource_name, resource_json, region_name):
        properties = resource_json['Properties']
        LOG.info('TODO: apigateway.Deployment.delete_from_cloudformation_json %s' % properties)
    if not hasattr(apigw_models.Deployment, 'delete_from_cloudformation_json'):
        apigw_models.Deployment.delete_from_cloudformation_json = depl_delete_from_cloudformation_json
    
    def vers_delete_from_cloudformation_json(
            resource_name, resource_json, region_name):
        properties = resource_json['Properties']
        LOG.info('TODO: apigateway.Deployment.delete_from_cloudformation_json %s' % properties)
    if not hasattr(lambda_models.LambdaVersion, 'delete_from_cloudformation_json'):
        lambda_models.LambdaVersion.delete_from_cloudformation_json = vers_delete_from_cloudformation_json
    
    @classmethod
    def RestAPI_create_from_cloudformation_json(cls, resource_name, cloudformation_json, region_name):