Added the ability to set knife_config_file to :auto

When the knife_config_file is set to :auto it will create a simple
knife.rb file with the chef values from the Vagrantfile. The auto
knife.rb will be created in a hidden cache folder waiting for the day
that you destroy the box. Once the box is destroyed the auto knife.rb
will be removed.

P.S. Please feel free to get picky with my ruby style since I usually write php.