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!
""" Convert data types in the "params" object, with the type defs
        specified in the 'types' attribute of "func_details". """
    types = func_details.get('types') or {}
    attr_names = types.keys() or []
    def cast(_obj, _type):
        if _type == bool:
            return _obj in ['True', 'true', True]
        if _type == str:
            return str(_obj)
        if _type == int:
            return int(_obj)
        return _obj
    def fix_types(o, **kwargs):
        if isinstance(o, dict):
            for k, v in o.items():
                if k in attr_names:
                    o[k] = cast(v, types[k])
        return o
    result = common.recurse_object(params, fix_types)
    return result
def remove_none_values(params):
    """ Remove None values recursively in the given object. """
    def remove_nones(o, **kwargs):