Fix markup on form-pay.php

Payment method fields were not displaying on the "Checkout -> Pay" page
because checkout.js was using a input#payment_method_{name} selector, which
worked fine with the payment method markup used in the review-order.php template,
but not with the markup used in the form-pay.php template.

This moves the #payment_method_{name} attribute from the <li/> elements to the
<input/> element so that checkout.js correctly display's payment method fields.
Merge pull request #4047 from thenbrent/form_pay_fix

Fix markup on form-pay.php