Work around selection_weight issue in form chooser

When the GatewayFormChooser finds multiple forms with equal
relevance to the country, currency, and payment method, it selects
based on selection_weight.  However, if all forms have zero
selection_weight, it throws them all out and returns an error.

This commit gives the main worldpay form a non-zero selection_weight
so the addition of equally-valid forms doesn't trigger the bug.

Change-Id: Ifc82f030c4364a2616613d521e4fffbff183e252