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.
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
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.
Joel Moss