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

  * /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
use str.format(); use correct variable
Merge pull request #12613 from plastikos/improvement-python_thin_shim

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