core: fix useDnsLookup behavior

This patch fixes the usage of "useDnsLookup" flag
at engine-manage-domains configuration.
only When --ldap-servers is used, this flag should be ignored if set to true.

Change-Id: Ic96e2f9fafcac1e6ba952a198bfb9cecc160ba8b
Bug-Url: https://bugzilla.redhat.com/1062172
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>