Fixes#3605 and adds the functionality suggested in PR #7440.
This PR is using a different appraoch that (IMHO) feels cleaner and (even more important) adds support for Windows at the same time.
See the updated docs for more details and examples, but in short this
enables the `attributes` param from the Chef provisioner to accept a
raw JSON string.
Fixes#3074Fixes#3572
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.