functional network tests: revert config on failure

With systemd, `systemctl vdsm-restore-net-config start` does nothing, as
that service is marked as already-started since boot time. Using
`restart` instead makes the tests revert config remnants upon test
failure.

Change-Id: I30dd957ebbbc2f5da069077a7713be27830fd24d
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/17097
Tested-by: Assaf Muller <amuller@redhat.com>
Reviewed-by: Assaf Muller <amuller@redhat.com>