webadmin: Changed bond list models to hold String

The models all used to hold VdsNetworkInterface entities, but only ever
used the NICs' names, so they really should have held String entities
all along. This will facilitate adding new bonds in following patches.

Change-Id: I4ede413e9c6fdc4b117eb10d6e7ff580f65324d1
Signed-off-by: Lior Vernia <lvernia@redhat.com>