engine: Update network referencing only attached clusters

Changed the query to be of the cluster attachments of the network, since
what we're interested in is only the clusters that are attached to the
network, both for checking VMs and for changing the network status.

Change-Id: I54a7ea672c1686f1a11f20ad06e076c84a7b1190
Signed-off-by: Mike Kolesnik <mkolesni@redhat.com>