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!
attribute='PhysicalResourceId')
                        except Exception as e:
                            LOG.debug('Unable to extract physical id for resource %s: %s' % (resource_id, e))
                else:
                    if callable(prop_key):
                        prop_value = prop_key(resource_props, stack_name=stack_name, resources=resources)
                    else:
                        prop_value = resource_props.get(prop_key)
                    if prop_value is not None:
                        params[param_key] = prop_value
    
    params = common.merge_recursive(defaults, params)
    
    for param_key, param_value in dict(params).items():
        if param_value is not None:
            param_value = params[param_key] = resolve_refs_recursively(stack_name, param_value, resources)
        
        if str(param_value).lower() in ['true', 'false']:
            params[param_key] = str(param_value).lower() == 'true'
    
    params = fix_account_id_in_arns(params)
    
    params = convert_data_types(func_details, params)
    
    params = remove_none_values(params)
    
    try:
        LOG.debug('Request for resource type "%s" in region %s: %s %s' % (
            resource_type, aws_stack.get_region(), func_details['function'], params))
        result = function(**params)