webadmin: Refactored Network QoS frontend code

Extracted the part concerned with the actual QoS values into
BaseNetworkQosModel and NetworkQoSpopupView, to be reusable from an
"anonymous" context (where name and DC are irrelevant).

Change-Id: I757d43302799cd5fc4c3006377a86c2841f39ea5
Signed-off-by: Lior Vernia <lvernia@redhat.com>