Per Brian Flad <bflad417@gmail.com>, switching chef_version toggle to be
provisioner/provisioner_version:

* Add provisioner variable and PROVISIONER environment variable
* Rename chef_version variable to provisioner_version
* Rename CHEF_VERSION environment variable to PROVISIONER_VERSION
* Rename chef.bat script to provisioner.bat
* Use PROVISIONER to determine action in provisioner.bat