IMPROVEMENT: Use Python for the thin shim instead of /bin/sh @thatch45

  * /bin/sh is, unfortunately, not very portable
  * Additional commands (tar, md5sum, etc.) are not uniform
  * Python *already* must be there to run salt and is very predictable
    and portable
  * All of the tools necessary to deploy thin are in Python stdlibs

This, unfortunately, discards the results of the shim when the shim succeeds - which is not horrible.  It would be nice if the shim output was always stored in the job cache.