(RAZOR-188) Add a refresh-repo command

This is useful in the following scenarios:
   - The content of the repo source has been updated and needs to updated on the razor server
   - A new razor server connected to an existing database (used by existing razor servers) is brought online.  This command will 'refresh' the repo onto the new server without wanting to manipulate the database.

This works by having the unpack_repo job remove the destination directory and recreate it before extracting the repo archive.  The refresh method simply publishes the make_repo_accessible job.