engine: Retrieve networks by label for a cluster

Adding the ability to fetch networks from the DB
with a specific label, where the networks are all
assigned to a specific cluster.

This will allow to identify which labelled networks
should be configured on the host when a nic is
labeled.

Change-Id: Ibffa2222669a42541cb2da80430d86c8aaddd471
Signed-off-by: Moti Asayag <masayag@redhat.com>