engine: Hide  vlan information of imported network

The vlan id of an imported network should not be exposed to the
engine, so it won't be persisted when the network is saved.

Change-Id: I972ff166c4cdefe5b84b28aead245a3fed2b10ba
Bug-Url: https://bugzilla.redhat.com/1031382
Signed-off-by: Moti Asayag <masayag@redhat.com>