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.

This patch includes a substantial fix based on
Iac6646bbad8c7431ec1c035ebc01f0180a8338ee by Antoni S. Puimedon.

Bug-Url: https://bugzilla.redhat.com/1059680
Change-Id: Ic8a5a933e1c93f80185d61f3cd43eee53c3a7c1a
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/23366
Reviewed-on: http://gerrit.ovirt.org/23792
Reviewed-on: http://gerrit.ovirt.org/23761
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>