Allow to disable TSO on L3 agent nodes

When running Linux kernel 3.14, TSO/GRO could be enabled on compute
nodes to improve performances.

To improve north-south performances, this feature is needed on Neutron
L3 agent nodes.

This patch add TSO management option for Neutron L3 agent and also
change the behavor for previous patch in ensuring that we use a kernel
>= 3.14 for both hypervisor & compute nodes.

Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>