DNS upgrade: change global forwarding policy in named.conf to "only" if private IPs are used
This change is necessary to override automatic empty zone configuration
in latest BIND and bind-dyndb-ldap 9.0+.
This upgrade has to be done on each IPA DNS server independently.
https://fedorahosted.org/freeipa/ticket/5710
Reviewed-By: Martin Basti <mbasti@redhat.com>