Cannot set session persistence on external keystone endpoint

[Bartosz Bezak]

Keystone federation using mod_auth_mellon needs to redirect back to the same keystone server. Therefore in multinode environment keystone external endpoint needs cookie for session persistence. Without it, mod_aut_mellon callback goes to endless loop.

as per - https://access.redhat.com/documentation/en-us/red_hat_openstack_platform/16.1/html/federate_with_identity_service/steps#step-18-use-proxy-persistence-for-keystone-on-each-controller