webadmin: Properly parameterize cluster ListModel

Parameterized the now-generic ListModel containing clusters in
NewNetworkModel, as well as the corresponding objects in the view
(which enabled to remove a few castings).

Change-Id: I1fedd156dc3a8e6d6defcf9260a3c66b6f9281db
Signed-off-by: Lior Vernia <lvernia@redhat.com>