[IMP] iap: generic method to recharge an account manually

Up until now, services developed (sms) relied on the get() and
InsufficientCreditError mecanism to create iap_accounts on the fly and
recharge it.

This particular workflow does not work well with automated processes which
is why we modify the function get_credit_url such that it will allow the
client to generate the 'recharge' link manually for better integration
of iap services.

addons/iap/models/iap.py

@@ -154,8 +154,10 @@ class IapAccount(models.Model):
         return account
 
     @api.model
-    def get_credits_url(self, base_url, service_name, credit):
+    def get_credits_url(self, service_name, base_url='', credit=0):
         dbuuid = self.env['ir.config_parameter'].sudo().get_param('database.uuid')
+        if not base_url:
+            base_url = get_endpoint(self.env)
         account_token = self.get(service_name).account_token
         d = {
             'dbuuid': dbuuid,