Werk #14548: mk_logwatch: Make agent plugin executable under Python 2

Werk 14451 introduced a regression which made the logwatch agent plugin crash under Python 2. The "Check_MK" service of affected hosts reported "Missing monitoring data for plugins: logwatch". The error message thrown by the plugin when executed directly on the host read

Traceback (most recent call last):
  ...
ValueError: invalid literal for int() with base 10: ...

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