Document chef provisioner config features: encrypted_data_bag_secret_key_path, log_level

I've added some basic documentation for those two features.  Please
verify that the examples work.  I wasn't sure exactly how to document
the `chef.encrypted_data_bag_secret` option, as it appears there have
been [some errors
encountered](https://groups.google.com/d/msg/vagrant-up/MncNoFCttgg/hHjmx4Ip54sJ)