engine: Avoid fetching null labels
engine: Avoid fetching null labels

It make sense to fetch only the defined labels
in the data center.

Change-Id: Ib068f8a87848dfc9845733a535e05a25934082f6
Signed-off-by: Moti Asayag <da1debb83a8e12b6e8822edf2c275f55cc51b720@redhat.com>

walteryang47/ovirt-engine,OpenUniversity/ovirt-engine,yapengsong/ovirt-engine,zerodengxinchao/ovirt-engine,yingyun001/ovirt-engine,halober/ovirt-engine,eayun/ovirt-engine,yapengsong/ovirt-engine,zerodengxinchao/ovirt-engine,yingyun001/ovirt-engine,zerodengxinchao/ovirt-engine,yingyun001/ovirt-engine,eayun/ovirt-engine,halober/ovirt-engine,walteryang47/ovirt-engine,OpenUniversity/ovirt-engine,eayun/ovirt-engine,OpenUniversity/ovirt-engine,OpenUniversity/ovirt-engine,yingyun001/ovirt-engine,OpenUniversity/ovirt-engine,halober/ovirt-engine,yapengsong/ovirt-engine,halober/ovirt-engine,walteryang47/ovirt-engine,zerodengxinchao/ovirt-engine,walteryang47/ovirt-engine,zerodengxinchao/ovirt-engine,yapengsong/ovirt-engine,yingyun001/ovirt-engine,eayun/ovirt-engine,eayun/ovirt-engine,walteryang47/ovirt-engine,yapengsong/ovirt-engine