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
This Adds three new arguments `use_policyfile`, `policy_group` and `policy_name` to the Chef
provisioner. If `use_policyfile` == true, then the other arguments are required.
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.