Make ConfdInotifyEventHandler a library function

Cut&Paste, plus the following changes:
  - The class is renamed to SingleFileEventHandler
  - The monitored filename must be passed in and doesn't default to the
    ganeti cluster config file
  - A small docstring is added to the class
  - Pylint disables for except: and method names are added

This makes it possible to write unittests for this class.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>