"Changing what is shipped" doesn't change but adds log paths

The [documentation](https://charmhub.io/filebeat) indicates that the operator can change the log files that are shipped with the 'logpath' config item:

```
Changing what is shipped
By default, the Filebeat charm will ship any container logs present in /var/lib/docker/containers as well as everything in:

/var/log/*/*.log
/var/log/*.log

If you'd rather target specific log files:

juju config filebeat logpath=/var/log/mylog.log
```
But the actual behaviour is that the contents of `logpath` are added to the configuration files, in addition to the defaults.
I believe that the charm should do as documented, which is to replace "what is shipped".

Read the Filebeat charm documentation at https://charmhub.io/filebeat and trace the handling of the `logpath` configuration item. Confirm how the default paths and configured paths are written to the configuration files; done means setting `logpath` replaces the defaults rather than adding to them.

Written by the indexing model from the issue text.