Versioned Endpoint hack for Sessions

To maintain compatibility we must allow people to specify a versioned
URL in the service catalog but allow the plugins to return a different
URL to users.

We need this to be a general approach as other services will likely have
a similar problem with their catalog.

The expectation here is that a client will register the catalog hack at
import time rather than for every request.

Closes-Bug: #1335726
Change-Id: I244f0ec3acca39fd1b2a2c5883abc06ec10eddc7