engine: Fix NPE in update network

Clusters field setting was mistakenly removed, putting it back in a form
that is not as ugly as it was before, but now update network won't cause
NPE :)

Change-Id: I9c56912740d8e790bb741cd2a70a7197d5f41be8
Signed-off-by: Mike Kolesnik <mkolesni@redhat.com>