webadmin: Vlan device is sent twice for SetupNetwork command

If vlan device with the same name as the new device already exists,
the new one should replace it (and not add both of them).

Change-Id: I0e444ea3064ea6fd6a1187e1e009e2ef6d2d54c6
Bug-Url: https://bugzilla.redhat.com/919052
Signed-off-by: Alona Kaplan <alkaplan@redhat.com>