webadmin: Don't attach external networks to NIC by label

External networks shouldn't be statically attached to a host's
interface, neither manually nor by label.

Change-Id: I202843bce1635facda3228855e8b2569370e4ed6
Bug-Url: https://bugzilla.redhat.com/1059233
Signed-off-by: Lior Vernia <lvernia@redhat.com>