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!
Default: python3.6
Resources:
  MyFunc:
    Type: AWS::Serverless::Function
    Properties:
      FunctionName: %s
      Handler: index.handler
      Runtime:
        Ref: LambdaRuntime
      InlineCode: |
        def handler(event, context):
            return {'hello': 'world'}
"""
def bucket_exists(name):
    s3_client = aws_stack.connect_to_service('s3')
    buckets = s3_client.list_buckets()
    for bucket in buckets['Buckets']:
        if bucket['Name'] == name:
            return True
def queue_exists(name):
    sqs_client = aws_stack.connect_to_service('sqs')
    queues = sqs_client.list_queues()
    try:
        url = name if '://' in name else aws_stack.get_sqs_queue_url(name)
    except Exception:
        return False
    for queue_url in queues['QueueUrls']:
        if queue_url == url:
            return True
def topic_exists(name):