MAGETWO-91704: Can't cancel/change payment method after chosen Store Credit

- Add locator for automated test

Author: lusinehak

app/code/Magento/Sales/Test/Mftf/Section/AdminOrderFormPaymentSection.xml

@@ -9,9 +9,10 @@
 <sections xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:noNamespaceSchemaLocation="urn:magento:mftf:Page/etc/SectionObject.xsd">
     <section name="AdminOrderFormPaymentSection">
+        <element name="useCustomerBalance" type="checkbox" selector="#p_method_use_customerbalance"/>
         <element name="header" type="text" selector="#order-methods span.title"/>
         <element name="getShippingMethods" type="text" selector="#order-shipping_method a.action-default" timeout="30"/>
         <element name="flatRateOption" type="radio" selector="#s_method_flatrate_flatrate" timeout="30"/>
         <element name="shippingError" type="text" selector="#order[has_shipping]-error"/>
     </section>
-</sections>
+</sections>