Allow to manage or not TSO issue

We need to allow some flexibility with TSO issue and allow the end-user
to disable the management of TSO.

Since TSO issue only affects GRE tunnels, when running VXLAN we should
be able to not manage TSO issue. So let's add a new parameter to manage
or not this feature.

Why not using facter to detect the kernel & using neutron flags to
detect the tunneling mode? Because we want to be sure that we have a
real flexibility and allow end-user to use a specific kernel (with a fix
or not) and make the choice to enable or not TSO/GSO.
Merge pull request #624 from enovance/tso-flexibility

Allow to manage or not TSO issue