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')
    response = cloudformation.describe_stack_resources(StackName=stack_name)
    for resource in response['StackResources']:
        if resource['LogicalResourceId'] == resource_logical_id:
            return resource
def list_stack_resources(stack_name):
    cloudformation = aws_stack.connect_to_service('cloudformation')
    response = cloudformation.list_stack_resources(StackName=stack_name)
    return response['StackResourceSummaries']
def get_queue_urls():
    sqs = aws_stack.connect_to_service('sqs')
    response = sqs.list_queues()
    return response['QueueUrls']
def get_topic_arns():
    sqs = aws_stack.connect_to_service('sns')
    response = sqs.list_topics()
    return [t['TopicArn'] for t in response['Topics']]
class CloudFormationTest(unittest.TestCase):
    def test_create_delete_stack(self):
        cloudformation = aws_stack.connect_to_resource('cloudformation')
        cf_client = aws_stack.connect_to_service('cloudformation')
        s3 = aws_stack.connect_to_service('s3')
        sns = aws_stack.connect_to_service('sns')
        apigateway = aws_stack.connect_to_service('apigateway')
        template = template_deployer.template_to_json(load_file(TEST_TEMPLATE_1))
        
        stack_name = 'stack-%s' % short_uid()
        cloudformation.create_stack(StackName=stack_name, TemplateBody=template)
        
        def check_stack():