Determine the Homebrew owner from enviroment variables & node attrs.

Homebrew installation and package commands will be run under a user
determined using the following rules (first match wins):

1. If node['homebrew']['owner'] is set, use this user
2. If ENV['SUDO_USER'] is present, Chef was invoked with sudo so
   use the original non-privileged user
3. Raise and exception if the calculated user is 'root'
4. Otherwise default to current user, taken from ENV['USER']