Fix ChefMirror referencing chef_server :client_key

I have been poking around with both of these LWRPs and found a few issues. 

The first is that `ChefResolvedCookbooks` was not added to the recipe_dsl.rb file and thus not available unless you explicitly require it. And the second is that `:client_key` is used to reference `chef_server` hash in mirror when we've used `:signing_key_filename` everywhere else!