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!
try:
            if attribute_name == 'StreamArn':
                streams = aws_stack.connect_to_service('dynamodbstreams').list_streams(TableName=self.name)['Streams']
                return streams[0]['StreamArn'] if streams else None
            return DynamoDB_Table_get_cfn_attribute_orig(self, attribute_name)
        except Exception as e:
            LOG.warning('Unable to get attribute "%s" from resource %s: %s' % (attribute_name, type(self), e))
            raise
    DynamoDB_Table_get_cfn_attribute_orig = dynamodb_models.Table.get_cfn_attribute
    dynamodb_models.Table.get_cfn_attribute = DynamoDB_Table_get_cfn_attribute
    
    def IAM_Role_get_cfn_attribute(self, attribute_name):
        try:
            return IAM_Role_get_cfn_attribute_orig(self, attribute_name)
        except Exception:
            if attribute_name == 'Arn':
                return aws_stack.role_arn(self.name)
            raise
    IAM_Role_get_cfn_attribute_orig = iam_models.Role.get_cfn_attribute
    iam_models.Role.get_cfn_attribute = IAM_Role_get_cfn_attribute
    
    def SNS_Topic_get_cfn_attribute(self, attribute_name):