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`.
By prefixing them with `cmd /c` it will work with both `winner` and
`ssh` connection types.
This PR also reverts some bad stringer changes made in PR #2673