Defer payment provider logout (bug 1037513)

The API to start a transaction doesn't return
a payment provider because the transaction has not
been created yet. This defers payment provider logout
until right before we redirect to the provider's web
flow.

@muffinresearch r?

There is also a Webpay patch coming shortly.
Something we should consider is looking at how we can move some of the lower-level (more detailed tests) into unit tests. That might me we can reduce the instances of things like this but for now this is a fairly common pattern across the uitests.