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 STATE.get('_thread_'):
        return STATE['_thread_']
    port = port or config.PORT_ELASTICSEARCH
    
    delete_all_elasticsearch_data()
    install.install_elasticsearch()
    backend_port = DEFAULT_PORT_ELASTICSEARCH_BACKEND
    es_data_dir = '%s/infra/elasticsearch/data' % (ROOT_PATH)
    es_tmp_dir = '%s/infra/elasticsearch/tmp' % (ROOT_PATH)
    es_mods_dir = '%s/infra/elasticsearch/modules' % (ROOT_PATH)
    if config.DATA_DIR:
        delete_data = False
        es_data_dir = '%s/elasticsearch' % config.DATA_DIR
    
    
    cmd = (('%s/infra/elasticsearch/bin/elasticsearch ' +
        '-E http.port=%s -E http.publish_port=%s -E http.compression=false ' +
        '-E path.data=%s') %
        (ROOT_PATH, backend_port, backend_port, es_data_dir))
    if os.path.exists(os.path.join(es_mods_dir, 'x-pack-ml')):
        cmd += ' -E xpack.ml.enabled=false'
    env_vars = {
        'ES_JAVA_OPTS': os.environ.get('ES_JAVA_OPTS', '-Xms200m -Xmx600m'),
        'ES_TMPDIR': es_tmp_dir
    }
    print('Starting local Elasticsearch (%s port %s)...' % (get_service_protocol(), port))