engine: Introduce GetNetworkLabelsByDataCenterIdQuery

The query returns all of the labels which are defined
in a specific data-center, either on networks or on
host nics.

Change-Id: I66a3f6b38c97d401ee4d7c59517b1e10ad8efb08
Signed-off-by: Moti Asayag <masayag@redhat.com>