universe should return opscode location type

Chef plans to redirect the cookbooks/community site URIs to supermarket in
order to facilitate the launch. Currently, returning
"supermarket" as the location type means that berkshelf (the client) cannot
download cookbooks from supermarket.

This change sets the location type to opscode by defining a new constant,
which can be updated at at later point if/when necessary.
If you're interested, it's possible to shorten `port.nil? || port.empty?` to `port.blank?`