By using OPENSTACK_KEYSTONE_URL instead of the HTTP_REFERRER
the authentication request between Horizon and Keystone continues
to work in situations where the HTTP_REFERRER is an external keystone
endpoint that Horizon does not have access to.
Change-Id: I9c5c8d59c5f5a8570dbb563ae224d45406a73ba5
Closes-bug: #1874705