Unified net persistence upgrade: Change 'bridged' from str to bool

'bridged' is defined as a bool in the VDSM API, although both
a string and a bool seem to work.

Change-Id: I13301d2b61e2e998e6623a8db3540ca6a8a6369c
Signed-off-by: Assaf Muller <amuller@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/22721
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>