core: set SingletonN __metaclass__ to ComputerI

Because my debug logs are flooded with messages like this:

DEBUG   [GLPI] Get manufacturers and their warranty urls
DEBUG   [GLPI] Get '__name__' warranty url
DEBUG   [GLPI] Get 'dell' warranty url
DEBUG   [GLPI] Get 'lenovo' warranty url
DEBUG   [GLPI] Get 'hp' warranty url
DEBUG   [GLPI] Get 'fujitsu' warranty url
DEBUG   [GLPI] Get 'toshiba' warranty url
DEBUG   [GLPI] Get 'apple' warranty url