[CHEF-4193] Enabling storage of roles in subdirectories

http://tickets.opscode.com/browse/CHEF-4193

Storing roles in subdirectories is already effectively enabled for Hosted Chef, as the directory structure gets flattened when they are uploaded. But it is not currently possible with Chef-Solo; one has to flatten the directory manually before running Chef-Solo. This patch changes that behavior, and enables Chef-Solo with roles in subdirectories.