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!
sns_client = aws_stack.connect_to_service('sns')
    topics = sns_client.list_topics()
    for topic in topics['Topics']:
        topic_arn = topic['TopicArn']
        if topic_arn.endswith(':%s' % name):
            return topic_arn
def queue_url_exists(queue_url):
    sqs_client = aws_stack.connect_to_service('sqs')
    queues = sqs_client.list_queues()
    return queue_url in queues['QueueUrls']
def stream_exists(name):
    kinesis_client = aws_stack.connect_to_service('kinesis')
    streams = kinesis_client.list_streams()
    return name in streams['StreamNames']
def get_stack_details(stack_name):
    cloudformation = aws_stack.connect_to_service('cloudformation')
    stacks = cloudformation.describe_stacks(StackName=stack_name)
    for stack in stacks['Stacks']:
        if stack['StackName'] == stack_name:
            return stack
def describe_stack_resource(stack_name, resource_logical_id):