netconfig: set ETHTOOL_OPTS when a NIC goes up

There's broken hardware out there where lro needs to to be turned off
explicitly.
This patch lets an admin of a local node set

  ethtool_opts.eth0 = lro off

to disable LRO on the interface named eth0.

Change-Id: Ic8a5a933e1c93f80185d61f3cd43eee53c3a7c1a
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/23366
Reviewed-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>