Support platform "suse" and "opensuse" on OpenSuSE

Fixes CHEF-4356: http://tickets.opscode.com/browse/CHEF-4356

A future update to Ohai (OHAI-339) will change the detected platform on
OpenSuSE from "suse" to "opensuse" to allow users to distinguish between
open and enterprise versions. Add provider mapping for "opensuse" such
that Chef will work correctly for both cases.