webadmin: Extracted common code in manage cluster networks

Code was essentially duplicated in both ClusterNetworkListModel and
NetworkClusterListModel, extracted it into the
ClusterNetworkManageModel for maintainability.

Change-Id: Ifa634f7d8011c25de7b1937984d4ec3627068b31
Signed-off-by: Lior Vernia <lvernia@redhat.com>