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 isinstance(template, string_types):
        template = parse_template(template)
    resource_map = template.get('Resources')
    if not resource_map:
        LOG.warning('CloudFormation template contains no Resources section')
        return
    next = resource_map
    iters = 10
    for i in range(0, iters):
        
        for resource_id, resource in next.items():
            stack_resource = describe_stack_resource(stack_name, resource_id)
            resource['__details__'] = stack_resource
        next = resources_to_deploy_next(resource_map, stack_name)
        if not next:
            return
        for resource_id, resource in next.items():
            deploy_resource(resource_id, resource_map, stack_name=stack_name)
    LOG.warning('Unable to resolve all dependencies and deploy all resources ' +
        'after %s iterations. Remaining (%s): %s' % (iters, len(next), next))
def delete_stack(stack_name, stack_resources):
    resources = dict([(r['LogicalResourceId'], common.clone_safe(r)) for r in stack_resources])
    for key, resource in resources.items():
        resources[key]['Properties'] = common.clone_safe(resource)
    for resource_id, resource in resources.items():
        delete_resource(resource_id, resources, stack_name)
def is_deployable_resource(resource):