Fix adapter lookup for external payment processors.

This fixes:

```

Time	2014/08/13 15:13:20.537926 GMT+2
User Name (User Id)	Anonymous User (None)
Request URL	http://test.zug.ch/behoerden/gemeinden/hunenberg/de/checkout
Exception Type	UnboundLocalError
Exception Value	local variable 'payment_processor' referenced before assignment
Traceback (innermost last):

Module ZPublisher.Publish, line 70, in publish
Module ZPublisher.mapply, line 77, in mapply
Module ZPublisher.Publish, line 48, in call_object
Module ftw.shop.browser.cart, line 482, in checkout
```


@lukasgraf 

I missed that one in a last commit...

thus also no changelog entry.