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 attribute_name in ['Arn', 'QueueArn']:
            return aws_stack.sqs_queue_arn(queue_name=self.name)
        return SQS_Queue_get_cfn_attribute_orig(self, attribute_name)
    SQS_Queue_get_cfn_attribute_orig = sqs_models.Queue.get_cfn_attribute
    sqs_models.Queue.get_cfn_attribute = SQS_Queue_get_cfn_attribute
    
    def S3_Bucket_get_cfn_attribute(self, attribute_name):
        if attribute_name in ['Arn']:
            return aws_stack.s3_bucket_arn(self.name)
        return S3_Bucket_get_cfn_attribute_orig(self, attribute_name)
    S3_Bucket_get_cfn_attribute_orig = s3_models.FakeBucket.get_cfn_attribute
    s3_models.FakeBucket.get_cfn_attribute = S3_Bucket_get_cfn_attribute
    
    @property
    def SQS_Queue_physical_resource_id(self):
        result = SQS_Queue_physical_resource_id_orig.fget(self)
        if '://' not in result:
            
            return aws_stack.get_sqs_queue_url(result)
        return result
    SQS_Queue_physical_resource_id_orig = sqs_models.Queue.physical_resource_id
    sqs_models.Queue.physical_resource_id = SQS_Queue_physical_resource_id
    
    def LogGroup_get_cfn_attribute(self, attribute_name):