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!
s3_client = aws_stack.connect_to_service('s3')
        value = b''
        if bucket_name != BUCKET_MARKER_LOCAL:
            value = s3_client.get_object(Bucket=bucket_name, Key=key_name)['Body'].read()
        return s3_models.FakeKey(name=key_name, value=value)
    s3_models.S3Backend.get_key = get_key
    
    def clean_json(resource_json, resources_map):
        result = clean_json_orig(resource_json, resources_map)
        if isinstance(result, BaseModel):
            if isinstance(resource_json, dict) and 'Ref' in resource_json:
                entity_id = get_entity_id(result, resource_json)
                if entity_id:
                    return entity_id
                LOG.warning('Unable to resolve "Ref" attribute for: %s - %s - %s',
                            resource_json, result, type(result))
        return result
    clean_json_orig = parsing.clean_json
    parsing.clean_json = clean_json
    
    parsing.MODEL_MAP.update(MODEL_MAP)
    
    def parse_and_create_resource(logical_id, resource_json, resources_map, region_name):