bridge_opts: Add ifcfg configurator support

bridge_opts is a new custom netwrok property that allows to tweak
the VM networks bridge behavior by passing a string such as:
    'max_age=2100 forward_delay=300'

This patch makes processing of such options part of vdsm proper by
including its processing in the ifcfg configurator.

Change-Id: I506538384ce4273630cf5fd18c56b07b09bd4376
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26708
Reviewed-by: Dan Kenigsberg <danken@redhat.com>