nagios: restructure check_ssl & misc fixes

- Restructure as a local package that has a run() that gets called if
  we're invoked directly or via the Icinga ePN (Python-__main__-style)
- Make the plugin ePN compatible but don't remove the -epn flags just
  yet as Icinga has troubles reloading the plugin when it gets changed.
- Minor reformatting across the board to make perl -w, perltidy &
  perlcritic happier.
- Print an informative message when we're returning an OK status.

Change-Id: I2ddda5bc7021d7ebcc4af44be2ebfcdfeadff24c