Marketplace purchase app using Cell data and Wifi workflows

This is a test for purchasing an app on Marketplace using the cell data workflow.

NB this contains part of the flow for WiFi payment as I had already written that before I realised cell data was different. I will follow this pull with a test for WiFi immediately.

Edit: I have added the test for adding the test Wifi. Note the change in the testvars_template.json . If we like this we will file another pull to change the remaining `this_phone_number` to be `carrier.phone_number` .

NBx2 : If you are outside of Spain, US and UK you may have problems testing this pull.

NBx3 : You also need to push the custom prefs after flashing your device:
https://webpay.readthedocs.org/en/latest/use_hosted_webpay.html#build-a-custom-b2g-profile