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!
cloudformation = aws_stack.connect_to_service('cloudformation')
        invalid_json = '{"this is invalid JSON"="bobbins"}'
        try:
            cloudformation.validate_template(TemplateBody=invalid_json)
            self.fail('Should raise ValidationError')
        except (ClientError, ResponseParserError) as err:
            if isinstance(err, ClientError):
                self.assertEqual(err.response['ResponseMetadata']['HTTPStatusCode'], 400)
                self.assertEqual(err.response['Error']['Message'], 'Template Validation Error')
    def test_list_stack_resources_returns_queue_urls(self):
        cloudformation = aws_stack.connect_to_resource('cloudformation')
        template = template_deployer.template_to_json(load_file(TEST_TEMPLATE_2))
        stack_name = 'stack-%s' % short_uid()
        cloudformation.create_stack(StackName=stack_name, TemplateBody=template)
        def check_stack():
            stack = get_stack_details(stack_name)
            self.assertEqual(stack['StackStatus'], 'CREATE_COMPLETE')
        retry(check_stack, retries=3, sleep=2)
        stack_summaries = list_stack_resources(stack_name)
        queue_urls = get_queue_urls()
        topic_arns = get_topic_arns()
        stack_queues = [r for r in stack_summaries if r['ResourceType'] == 'AWS::SQS::Queue']
        for resource in stack_queues:
            self.assertIn(resource['PhysicalResourceId'], queue_urls)
        stack_topics = [r for r in stack_summaries if r['ResourceType'] == 'AWS::SNS::Topic']
        for resource in stack_topics:
            self.assertIn(resource['PhysicalResourceId'], topic_arns)
    def test_create_change_set(self):