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!
return o if o is None else json.dumps(o) if isinstance(o, (dict, list)) else str(o)
def select_attributes(obj, attrs):
    result = {}
    for attr in attrs:
        if obj.get(attr) is not None:
            result[attr] = str_or_none(obj.get(attr))
    return result
def get_bucket_location_config(**kwargs):
    return {'LocationConstraint': aws_stack.get_region()}
def lambda_get_params():
    return lambda params, **kwargs: params
def get_lambda_code_param(params, **kwargs):
    code = params.get('Code', {})
    zip_file = code.get('ZipFile')
    if zip_file and not common.is_base64(zip_file):
        tmp_dir = common.new_tmp_dir()
        handler_file = get_handler_file_from_name(params['Handler'], runtime=params['Runtime'])
        tmp_file = os.path.join(tmp_dir, handler_file)
        common.save_file(tmp_file, zip_file)
        zip_file = create_zip_file(tmp_file, get_content=True)
        code['ZipFile'] = common.to_str(base64.b64encode(zip_file))
        code['ZipFile'] = zip_file
        common.rm_rf(tmp_dir)
    return code
RESOURCE_TO_FUNCTION = {
    'S3::Bucket': {
        'create': {
            'function': 'create_bucket',
            'parameters': {
                'Bucket': ['BucketName', PLACEHOLDER_RESOURCE_NAME],