[FIX] sale: fix call _find_mail_template on multiple record

The implementation of _find_mail_template allow to call it only
on one record at a time.

This was not the case with in _send_order_confirmation_mail.

- Add excplicit constraint in _find_mail_template
- Make sure _send_order_confirmation_mail call the method once per
  record

closes odoo/odoo#102135

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>