[py3] Let InstanceCommand.run_arg() always return an int The caller of this method (which is method "run_args") does `max(status, self.run_arg(appid))` and run_arg might return None if the underlying <cmdname>_instance method returns None. On python3 max(1, None) breaks with "TypeError: unorderable types: int() > NoneType()". So we enforce run_arg() method to return an integer even if the underlying method returns None. This fixes call of "cubicweb-ctl i18ninstance" with Python3.

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