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!
def fix_ids(o, **kwargs):
        if isinstance(o, dict):
            for k, v in o.items():
                if common.is_string(v, exclude_binary=True):
                    o[k] = aws_stack.fix_account_id_in_arns(v)
        return o
    result = common.recurse_object(params, fix_ids)
    return result
def convert_data_types(func_details, params):