A somewhat icky way of only managing the key once.

The `puppet_enterprise::license` manages the license file on the
master, but only after it exists. So we do the opposite. We manage
the file, but only when it doesn't exist.

On first run, we'll manage it, but on subsequent runs PE will take over.
Merge pull request #157 from binford2k/ticket/TRAINVM-42/fix_license_file_management

A somewhat icky way of only managing the key once.