In the current state, it seems we can only send Slack or Mattermost notifications in one specific channel.
The webhook string is usually pointing to a channel or we can set the
`org.opennms.netmgt.notifs.slack.channel` option in `slack.properties`.
Both ways only send into this one defined channel.
In practice it's not handy, since you have 1-n teams who need alarm notifications for their environment in their team channel.
From OpenNMS point of view it would be the easiest way, to make the channel option usable in a destination path.
With that it would be possible to send notifications into different channels.