ensure filebeat doesn't sync logsink.log files for juju controllers

[Garrett Neugent]

logsink.log files come from Juju controllers, and are duplicating log entries from individual nodes. As such, they do not need to be synced with filebeat, and when synchronized to a back end (like Graylog), they take a lot of space.

When configured correctly in the charm config, "logsink.log" would be present in the exclude_files setting. This setting would only apply to Juju controllers, so I Juju-lint should handle checking this for newly-deployed clouds, rather than a charm-wide default.