The keystone API version 3 has become the default API version
for a long time and our CI already uses keystone v3 API only.
In addition, our documentation recommended to use the cached session,
so switching the default API version to 3 does not hit the limitation
on the maximum size of session data when using the cookie-based
sessions.
This is based on django_opensatck_auth review
https://review.openstack.org/#/c/158384/
Implements blueprint: default-keystone-api-3
Change-Id: I79e9fbd3567cc69468694dbf043fecf422347c7e
cd49510878