webadmin:  Editing non-VLAN network shows VLAN 0

EditNetworkModel translates 'Null' vlan id to 0.
It shouldn't do it.

Change-Id: I9ea207d0a881500de4fe5a633e6fba5f9179ea3d
Bug-Url: https://bugzilla.redhat.com/1114985
Signed-off-by: Alona Kaplan <alkaplan@redhat.com>
(cherry picked from commit 5bbea9650e2850974c5417c96d42c33b0d97a5f9)