use keystone public endpoint in horizon
use keystone public endpoint in horizon

the horizon::keystone_url is ultimately used to set the
OPENSTACK_KEYSTONE_URL setting in Horizon's local_settings, and this
is used in browser redirects when utilizing web SSO. In many cases,
the keystone internal endpoint would be inaccessible to browser
clients, so we should use the public endpoint here.

Change-Id: I5b3c0935b1a5c38704e748770b7bac52d674a637

openstack/tripleo-heat-templates,dprince/tripleo-heat-templates,dprince/tripleo-heat-templates,openstack/tripleo-heat-templates