webadmin: Modified model logic to support adding new bonds

The model used to check that there were actually free bonds reported by
the host; this is no longer necessary once we enable the user to add new
bonds. Also deleted the corresponding error message.

Change-Id: I7d3c2377ff9d95dfff1c3c416e7fc9242795e227
Signed-off-by: Lior Vernia <lvernia@redhat.com>