ifcfg: set onboot=no when persistence is 'unified'

Up until now the ifcfg network configurator had been setting up
onboot=yes so that when starting up the computer, network.service
would bring up the vdsm networks and devices. This, however, is
counter productive when using the 'unified' net_persistence setting
as the boot up process would:
1. network.service would create the devices and networks
2. vdsm-restore-net-config would remove all the networks
3. vdsm-restore-net-config would flush the ifcfg files
4. vdsm-restore-net-config would bring up the networks and devices
   again

In the case of dhcp, that would imply a lot of duplicated address
assignment time. Thus, this patch eliminates the step one from
above.

Change-Id: I1ad8bbdcaeb1790b7955f3a407758ee41c68befa
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27541
Reviewed-by: Dan Kenigsberg <danken@redhat.com>