Fixes an issue that crept in with d782a99 where auth_token
started defaulting to the v3.0 API by default when no version
was specified.
Given that bin/keystone still defaults to using the v2.0 API
it seems like auth_token.py should too.
Fixes LP Bug #1154144
Change-Id: Ia5620bccc182bbc73cb60dcccb1f701304450e5a