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!
if delete_data:
        rm_rf(es_data_dir)
    
    chmod_r('%s/infra/elasticsearch' % ROOT_PATH, 0o777)
    mkdir(es_data_dir)
    chmod_r(es_data_dir, 0o777)
    mkdir(es_tmp_dir)
    chmod_r(es_tmp_dir, 0o777)
    
    start_proxy_for_service('elasticsearch', port, backend_port,
        update_listener, quiet=True, params={'protocol_version': 'HTTP/1.0'})
    if is_root():
        cmd = "su localstack -c '%s'" % cmd
    thread = do_run(cmd, asynchronous, env_vars=env_vars)
    STATE['_thread_'] = thread
    return thread
def check_elasticsearch(expect_shutdown=False, print_error=False):
    out = None
    try:
        
        es = aws_stack.connect_elasticsearch()
        out = es.cat.aliases()
    except Exception as e:
        if print_error:
            LOG.error('Elasticsearch health check failed (retrying...): %s %s' % (e, traceback.format_exc()))
    if expect_shutdown:
        assert out is None
    else:
        assert isinstance(out, six.string_types)