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!
stack = get_stack_details(stack_name)
            self.assertEqual(stack['StackStatus'], 'CREATE_COMPLETE')
        retry(check_stack, retries=3, sleep=2)
        
        assert bucket_exists('cf-test-bucket-1')
        assert queue_exists('cf-test-queue-1')
        topic_arn = topic_exists('%s-test-topic-1-1' % stack_name)
        assert topic_arn
        assert stream_exists('cf-test-stream-1')
        resource = describe_stack_resource(stack_name, 'SQSQueueNoNameProperty')
        assert queue_exists(resource['PhysicalResourceId'])
        
        tags = s3.get_bucket_tagging(Bucket='cf-test-bucket-1')['TagSet']
        self.assertEqual(tags, [{'Key': 'foobar', 'Value': aws_stack.get_sqs_queue_url('cf-test-queue-1')}])
        tags = sns.list_tags_for_resource(ResourceArn=topic_arn)['Tags']
        self.assertEqual(tags, [
            {'Key': 'foo', 'Value': 'cf-test-bucket-1'},
            {'Key': 'bar', 'Value': aws_stack.s3_bucket_arn('cf-test-bucket-1')}