restapi: Allow untagging a network with vlan-id

The patch allows untagging a network which has a vlan-id
configured by providing the vlan element without specifying
its id, e.g. via PUT:

<network>
    <vlan />
    ...
</network>

Change-Id: Id200dedd19412add194133a6f6554ecf6797f5a8
Bug-Url: https://bugzilla.redhat.com/970635
Signed-off-by: Moti Asayag <masayag@redhat.com>