Add --ssl-verify-mode and --[no-]verify-api-cert options.

Resolves [CHEF-5282](https://tickets.opscode.com/browse/CHEF-5282), and first on the list for https://github.com/opscode/chef/issues/1542.
I think this logic won't get triggered if you set the value in your knife.rb. Might want to move it into a method that we call from the `#run` method?