Builds on the work of #3846, shifting the Chef provisioner's
configuration options from `secret_key_path` and `validation_key_path`
over to `secret_key` and `validation_key`.
Before this option (`os_type`) the provisioner would use the connection
type to determine the targeted OS. When not supplying a value for
`os_type`, it will fall back to the old behaviour, so this is full BC.
Paul Hinze