Use attr to define start view (bug 1036935)

This allows webpay to define a starting view in the SPA.

This means we can modify the existing provider views to just serve the SPA along with a data attr to tell Spartacus what view should be used.
this is a little weird. I'd suggest a comment like "This simulates one of many possible URLs that Webpay could serve a wait-to-finish page from"