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`.
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