core: Fix multiple domains to support useDnsLookup flag

Even if useDnsLookup flag is set to true at
/etc/ovirt-engine/engine-manage-domains/engine-manage-domains.conf
The [domain_realm] with domain realm mapping must exist in case there is more than one managed domain.

Change-Id: Iebad744a40370c970d9462fb628092d9ebed13b1
bug-url: https://bugzilla.redhat.com/905291
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>