Allow full path to environment.

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.
Use use_subdomain attr instead of magic env name.

Environment _default was previously magic. Now there's an
explicit attribute use_subdomain which must be set to
false to get the magic.
Merge pull request #7 from Metaswitch/fix-dogfood

[Reviewer: Andy] Fix dogfood