Clone

This pull request is meant to solicit discussion of a proof of concept clone feature built into vmware and virtualbox builders.
This will take the output dir data from one packer run and use it as the source of a second run.  This packer run 1 builds the base os.  The next run/json will create the customized box.

"builders": [
    {
      "type": "vmware",
      "source_vmx_path": "/output-vmware/ubuntu1210.vmx",
    },
    {
      "type": "virtualbox",
      "source_ovf_file": "/output-virtualbox/packer.ovf",
    }
]