In fact I realized that the class res.config.settings is of type osv_memory meaning that lines in the database are not kept ! The correct way of using this class is to name fields 'default_xxx'. The execute method will store the default value in the ir_values table so that we can retrieve them by calling the default_get method on the model object. Here is an example :

class mysql_config(osv.osv_memory):
    _inherit = 'res.config.settings'
    _name = 'mysql.config.settings'
    
    _columns = {
        'default_mysql_host' : fields.char('MySQL Host', default_model='mysql.config.settings')
    }