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!
try:
            return LogGroup_get_cfn_attribute_orig(self, attribute_name)
        except Exception:
            if attribute_name == 'Arn':
                return aws_stack.log_group_arn(self.name)
            raise
    LogGroup_get_cfn_attribute_orig = getattr(cw_models.LogGroup, 'get_cfn_attribute', None)
    cw_models.LogGroup.get_cfn_attribute = LogGroup_get_cfn_attribute
    
    def Lambda_Function_get_cfn_attribute(self, attribute_name):
        try:
            if attribute_name == 'Arn':
                return self.function_arn
            return Lambda_Function_get_cfn_attribute_orig(self, attribute_name)
        except Exception:
            if attribute_name in ('Name', 'FunctionName'):
                return self.function_name
            raise
    Lambda_Function_get_cfn_attribute_orig = lambda_models.LambdaFunction.get_cfn_attribute
    lambda_models.LambdaFunction.get_cfn_attribute = Lambda_Function_get_cfn_attribute
    
    def DynamoDB_Table_get_cfn_attribute(self, attribute_name):