engine: Added IsDisplayAddressConsistentInClusterQuery

This query returns true if the display address is overridden for some hosts in
a cluster and for some not. Returns false othervise.

It is needed by the http://wiki.ovirt.org/Features/Display_Address_Override to
avoid this calculation on the frontend.

Change-Id: I1955c35a3739680f6f80c9e8b29a21a14c5b8b7a
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>