Add option to install Puppet as a provisioner on CentOS boxes

I've added Puppet as an option, using the Puppet Labs yum repositories. To do so, I've added a couple of configuration variables to the packer templates for each CentOS release and updated the provisioner script.

I'm starting the work to add it to other *nix variants with repositories (Fedora, Debian, Ubuntu) next but since I needed CentOS right now I thought I'd open a pull request and get the ball rolling.