tools: In case ldapServers is used, useDnsLookup should be ignored

As a part of introduction of -ldapSerers option in manage-domains,
useDnsLookup flag must be ignored, so KDC list appears in krb5.conf file
and the KDCs will be selected in the order of appearance in krb5.conf
This patch has an assumption in which ldap servers serve as KDC.

Bug-Url: https://bugzilla.redhat.com/894681
Change-Id: I808fe02553b5554b76f08b43a30fb0ffd7d99931
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>