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!
""" Convert a CloudFormation ACL string (e.g., 'PublicRead') to an S3 ACL string (e.g., 'public-read') """
    return re.sub('(?<!^)(?=[A-Z])', '-', acl).lower()
def retrieve_topic_arn(topic_name):
    topics = aws_stack.connect_to_service('sns').list_topics()['Topics']
    topic_arns = [t['TopicArn'] for t in topics if t['TopicArn'].endswith(':%s' % topic_name)]
    return topic_arns[0]
def get_role_arn(role_arn, **kwargs):
    role_arn = resolve_refs_recursively(kwargs.get('stack_name'), role_arn, kwargs.get('resources'))
    return aws_stack.role_arn(role_arn)
def parse_template(template):
    try:
        return json.loads(template)
    except Exception:
        yaml.add_multi_constructor('', moto.cloudformation.utils.yaml_tag_constructor, Loader=NoDatesSafeLoader)
        try:
            return yaml.safe_load(template)
        except Exception:
            return yaml.load(template, Loader=NoDatesSafeLoader)
def template_to_json(template):
    template = parse_template(template)
    return json.dumps(template)
def get_resource_type(resource):
    res_type = resource.get('ResourceType') or resource.get('Type') or ''
    parts = res_type.split('::', 1)
    if len(parts) == 1:
        return None
    return parts[1]
def get_service_name(resource):