[Reviewer: Andy] Fix dogfood

Hi Andy - please can you review.

Two commits here:

*  Allow calling update_chef_server.sh with a full path, rather than
   just an environment name. That allows us to have a separate
   directory (from a separate repo) containing the environments, so
   they don't have to be checked into the same repo.

*  Environment _default was previously magic. Now there's an
   explicit attribute use_subdomain which must be set to
   false to get the magic.

I've tested the first works, but I've not tested the second yet.

Thanks.

--KW 8-)