Add net attribute, deprecate networking

--networking has been replaced by --net=X in Docker 0.11. This attempts to fix #150

This is the minimum viable fix for this. I have some other ideas but want to run them past smarter people first.

1. Should we validate that net is one of `bridge, host, container:, or none`? This makes things slightly harder to mess up but will require updates if more options appear in the future.

1. We should probably make `net` or `networking` available conditionally on the Docker version but at least as of now I didn't see a way to check that. Probably a worthwhile feature for the cookbook in general? I can try to implement that in a separate PR (would require determining that both client and server are >x.x).