Add warning about GRO and Neutron routers

Generic Receive Offload appears to be enabled by default on recent Ubuntu
kernels. It can have a significant impact on download performance when
enabled on a Neutron router interface. This patch warns users about that.

Change-Id: I3d3a560b1db55aabd901f27ad5c7bd5777b300da
Closes-bug: 1252900
(cherry picked from commit 3cc8efdf5466750334e912ae8efa4cc8c0354edb)