tools: Fix LDAP servers issue in engine-manage-domains

LDAP servers discovered using DNS query will not be written to database
(they will be rediscovered using DNS query when needed). LDAP servers
entered in --ldap-servers argument will be always written to database
(they will be used then and no LDAP DNS discovery will be executed).

Change-Id: I4ab5bd3a27787615c7430ce2f3a120261f4cd300
Bug-Url: https://bugzilla.redhat.com/1084479
Signed-off-by: Martin Perina <mperina@redhat.com>