webadmin: Fix type-casting warnings in ClusterManageNetworkPopupView
and re-arrange it.

In ClusterManageNetworkPopupView class:
   1. Change all generic EntityModel occurencies to a specific model
   class (i.e. ClusterNetworkModel).
   2. Remove all type type castings.
   3. Convert anonymous classes into inner ones.
Define ClusterNetworkModel class as an extetion of EntityModel<Network>
(add Network as a type parameter).

Change-Id: If0a72f5530388d1abb6758e0d9cc9e5c8ebdb3e9
Signed-off-by: Yevgeny Zaspitsky <yzaspits@redhat.com>