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 ref == 'AWS::Region':
        return aws_stack.get_region()
    resource_status = {}
    if stack_name:
        resource_status = describe_stack_resource(stack_name, ref)
        if not resource_status:
            return
        attr_value = resource_status.get(attribute)
        if attr_value not in [None, '']:
            return attr_value
    elif ref in resources:
        resource_status = resources[ref]['__details__']
    
    resource = resources.get(ref)
    resource_new = retrieve_resource_details(ref, resource_status, resources, stack_name)
    if not resource_new:
        return
    resource_type = get_resource_type(resource)
    result = extract_resource_attribute(resource_type, resource_new, attribute)
    if not result:
        LOG.warning('Unable to extract reference attribute %s from resource: %s' % (attribute, resource_new))
    return result
def resolve_refs_recursively(stack_name, value, resources):