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!
try:
            return _parse_and_create_resource(logical_id, resource_json, resources_map, region_name)
        except Exception as e:
            LOG.error('Unable to parse and create resource "%s": %s %s' %
                      (logical_id, e, traceback.format_exc()))
            raise
    def parse_and_update_resource(logical_id, resource_json, resources_map, region_name):
        try:
            return _parse_and_create_resource(logical_id,
                resource_json, resources_map, region_name, update=True)
        except Exception as e:
            LOG.error('Unable to parse and update resource "%s": %s %s' %
                      (logical_id, e, traceback.format_exc()))
            raise
    def _parse_and_create_resource(logical_id, resource_json, resources_map, region_name, update=False):
        stack_name = resources_map.get('AWS::StackName')
        resource_hash_key = (stack_name, logical_id)
        
        updating = CURRENTLY_UPDATING_RESOURCES.get(resource_hash_key)
        LOG.debug('Currently updating stack resource %s/%s: %s' % (stack_name, logical_id, updating))
        if updating:
            return None