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!
resource_type = get_resource_type(resource)
    service = get_service_name(resource)
    resource_config = RESOURCE_TO_FUNCTION.get(resource_type)
    if resource_config is None:
        raise Exception('CloudFormation deployment for resource type %s not yet implemented' % resource_type)
    try:
        if func_config.get('boto_client') == 'resource':
            return aws_stack.connect_to_resource(service)
        return aws_stack.connect_to_service(service)
    except Exception as e:
        LOG.warning('Unable to get client for "%s" API, skipping deployment: %s' % (service, e))
        return None
def describe_stack_resource(stack_name, logical_resource_id):