Common keystone version fallback

If OPENSTACK_API_VERSIONS["identity"] was not set in local_settings.py,
django_openstack_auth and horizon had their own fallback method which
did not necessarily fallback on the same version and was leading to
discrepancies and erratic behavior.

Refactored in order to use the same fallback method defined in
django_openstack_auth.utils

Change-Id: I583d49a73077a20baf5f01628b0b6e8480296964
Closes-Bug: #1231357
Merge "Common keystone version fallback"