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(obj, **kwargs):
        if isinstance(obj, dict):
            obj = dict(obj)
            for key, value in obj.items():
                if isinstance(value, BaseModel):
                    entity_id = get_entity_id(value)
                    obj[key] = entity_id or value
        return obj
    return recurse_object(resource_json, fix_ids)
def update_physical_resource_id(resource):
    phys_res_id = getattr(resource, 'physical_resource_id', None)
    if not phys_res_id:
        if isinstance(resource, lambda_models.LambdaFunction):
            func_arn = aws_stack.lambda_function_arn(resource.function_name)
            resource.function_arn = resource.physical_resource_id = func_arn
        elif isinstance(resource, sfn_models.StateMachine):
            sm_arn = aws_stack.state_machine_arn(resource.name)
            resource.physical_resource_id = sm_arn
        elif isinstance(resource, service_models.StepFunctionsActivity):
            act_arn = aws_stack.stepfunctions_activity_arn(resource.params.get('Name'))
            resource.physical_resource_id = act_arn
        elif isinstance(resource, kinesis_models.Stream):
            resource.physical_resource_id = resource.stream_name
        else:
            LOG.warning('Unable to determine physical_resource_id for resource %s' % type(resource))
def apply_patches():
    """ Apply patches to make LocalStack seamlessly interact with the moto backend.
        TODO: Eventually, these patches should be contributed to the upstream repo! """
    
    def get_key(self, bucket_name, key_name, version_id=None):