[logging] allow custom message format

## preface
I'm using _conmon_ with _podman_. Since some containerized applications don't have good logging support, I'm using _journald_ to capture the logs from _conmon_. For longtime storage, I'm processing the logs using _rsyslog_.

## What's my issue?
Although _conmon_ exported the container name and tags into _journld_, these details aren't part of the _syslog_ export. It's impossible to filter out by container name. So _rsyslog_ has to use the _imjournal_ module, which makes all the tags available - but with way trouble in performance.

```
Jun 6 20:00:00 debian conmon[12975]: 2021-06-06 20:00:00.000 UTC [2] LOG: database system is shut down
````

## What might be a possible solution?
Allow configuring the _syslog_ message.

```
Jun 6 20:00:00 debian conmon[12975]: $CONTAINER-NAME 2021-06-06 20:00:00.000 UTC [2] LOG: database system is shut down
```

or

```
Jun 6 20:00:00 debian conmon-$CONTAINER-NAME[12975]: 2021-06-06 20:00:00.000 UTC [2] LOG: database system is shut down
```

etc.