filebeat gets restarted on every hook

[axinojolais]

There's a pretty nasty bug hidden in this layer. Here's how it goes, please bear with me :

1. render_without_context() is called (from the "main" charm, typically - filebeat in my case)
2. context = config() is run. context is now a reference to a Config object.
3. If you have set the logpath config option, then context['logpath'] = context['logpath'].split(' ') is run, which modifies context
4. Because the Config class constructor contains atexit(self._implicit_save) (and because the implicit_save class attribute of the Config class defaults to true), the modified context is saved to disk as the new config
5. When the next hook is run, the basic layer compares the current config and the config saved to disk, finds they're different, and sets the config.changed flag
6. This layer sets beat.render
7. The charm renders the config and restarts filebeat

I'm submitting a PR to fix.

[axinojolais]

This also resolves canonical/layer-filebeat#25