DNS server upgrade: do not fail when DNS server did not respond
    
    Previously, update_dnsforward_emptyzones failed with an exeception if
    DNS query failed for some reason. Now the error is logged and upgrade
    continues.
    
    I assume that this is okay because the DNS query is used as heuristics
    of last resort in the upgrade logic and failure to do so should not have
    catastrophics consequences: In the worst case, the admin needs to
    manually change forwarding policy from 'first' to 'only'.
    
    In the end I have decided not to auto-start BIND because BIND depends on
    GSSAPI for authentication, which in turn depends on KDC ... Alternative
    like reconfiguring BIND to use LDAPI+EXTERNAL and reconfiguring DS to
    accept LDAP external bind from named user are too complicated.
    
    https://fedorahosted.org/freeipa/ticket/6205
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>