[FIX] payment: payment reference length

The payment reference of a S2S transaction has the given format:
'PAYMENT-9-170110_200204'. However, the reference is too long for some
providers such as Authorize (limited to 20 characters).

We make the reference less verbose as a first step to solve the issue.
However, case-by-case solutions will need to be found since the
reference depends on the id, which will ultimately grow.

opw-704615

addons/payment/models/account_payment.py

@@ -41,7 +41,7 @@ class AccountPayment(models.Model):
         if self.payment_token_id.acquirer_id.auto_confirm == 'authorize':
             raise ValidationError('This feature is not available for payment acquirers set to the "Authorize" mode.\n'
                                   'Please use a token from another provider than %s.' % self.payment_token_id.acquirer_id.name)
-        reference = "PAYMENT-%s-%s" % (self.id, datetime.datetime.now().strftime('%y%m%d_%H%M%S'))
+        reference = "P-%s-%s" % (self.id, datetime.datetime.now().strftime('%y%m%d_%H%M%S'))
         tx = self.env['payment.transaction'].create({
             'amount': self.amount,
             'acquirer_id': self.payment_token_id.acquirer_id.id,