Add kitchen provisioner for policyfiles w/ chef zero

:construction: **for this to work, we need a TK release that supports mixlib-shellout 2.x**

The provisioner is in a working state, though I plan to update the `chef export` command's tests for conflicting files to make them less strict. In particular, `chef export` will complain/fail only if there is a conflicting `cookbooks/` or `data_bags/policyfiles` directory.

Any comments in the interim are welcome, however. @opscode/client-engineers 
Good catch, that's leftover from the Chef Zero provisioner I copied this from.