Use v2.0 api by default in auth_token middleware

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
Merge "Use v2.0 api by default in auth_token middleware"