webadmin: Fixed bond-related flows in Setup Networks

Previously the free bonds on a host were queried every time the Setup
Networks dialog was redrawn, where they could in fact only change when
backend data was retrieved. Moreover, the computation of the next free
bond name was performed by mistake several times whenever the dialog
was redrawn, instead of just once.

Change-Id: I9eacb27404eca28bb890973545f08fec9b983c4e
Signed-off-by: Lior Vernia <lvernia@redhat.com>