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

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.