ifcfg: make default route network be started by sysV

With the change to unified persistence, the networks defined by vdsm
are only started and restored on boot by vdsmd itself. The issue with
this is that if vdsmd fails to start for whichever reason, that may
impede remote access to the machine.

This patch is a fix for oVirt 3.5 only (future versions will have
separate network and vdsm initializations) that solves the issue by
making all the interfaces that are needed for the network that is
defined to carry the default route (typically the management network)
have ONBOOT=yes. That setting makes the initscripts network service
configure the network itself. So for the default route network, the
boot process is going to be:
    a) sysV or systemd start network.service,
    b) network.service goes over all the ifcfg files and ifups those
       with ONBOOT=yes
    c) vdsmd start will call vdsm-network-restore-config which will:
        + flush ifcfg files
        + call setupNetworks to restore all the defined networks.

It's easy to see that if (c) does not happen we will be left with
only the default route network, so remote access will still be
possible.

Bug-Url: https://bugzilla.redhat.com/1128140
Change-Id: Iaf656958e8ddfdd7e8be9a8bcb51962e073ae26a
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/29312
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>