Determines version from auth_url when not explicit

This changes the default version to None instead
of '3', and grabs the version from the auth_url.
Before, passing it only a v2 auth_url would break
with no good explanation.

Change-Id: I97b22ee9b978c34e98c220441fbea940b76e8e5f
Co-Authored-By: Telles Mota Vidal Nóbrega <tellesmvn@lsd.ufcg.edu.br>
Closes-Bug: 1357109
Merge "Determines version from auth_url when not explicit"