Adding account vat on payment 7

Moved from https://code.launchpad.net/~domsense/account-payment/adding_account_vat_on_payment_7/+merge/196914
There are too many spaces between a and VAT here. You may want to do:
```python
            help='Related account used for real registrations on a '
                 'VAT on payment basis. Set the shadow account here'
```