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!
if isinstance(value, dict):
        keys_list = list(value.keys())
        
        if keys_list == ['Ref']:
            result = resolve_ref(stack_name, value['Ref'],
                resources, attribute='PhysicalResourceId')
            return result
        if keys_list and keys_list[0].lower() == 'fn::getatt':
            return resolve_ref(stack_name, value[keys_list[0]][0],
                resources, attribute=value[keys_list[0]][1])
        if keys_list and keys_list[0].lower() == 'fn::join':
            return value[keys_list[0]][0].join(value[keys_list[0]][1])
        if keys_list and keys_list[0].lower() == 'fn::sub':
            result = value[keys_list[0]][0]
            for key, val in value[keys_list[0]][1].items():
                val = resolve_refs_recursively(stack_name, val, resources)
                result = result.replace('${%s}' % key, val)
            return result
        else:
            for key, val in iteritems(value):
                value[key] = resolve_refs_recursively(stack_name, val, resources)
    if isinstance(value, list):
        for i in range(0, len(value)):
            value[i] = resolve_refs_recursively(stack_name, value[i], resources)
    return value
def update_resource(resource_id, resources, stack_name):
    resource = resources[resource_id]
    resource_type = get_resource_type(resource)
    if resource_type not in UPDATEABLE_RESOURCES: