Move Ethernet device speed finding to the Network cookbook.

This moves the code we use to detect what speeds an Ethernet device
supports out of ohai and in to the network cookbook.  This prevents
everyone from paying for the additional network round trips that were 
potentially needed to pull in the cstruct gem when we did not need it,
and places the speed detection code with its only user.

The speed detection code now also knows how to detect 2.5g, 20g and 40g
links for the proud owners of such exotic hardware.
The kernel guarantees that the contents of that file will always be an integer in string form.