Checkmk Werk 7440: Added Mattermost support in notification plugin

ID: 7440 Title: Added Mattermost support in notification plugin Component: Notifications Level: 1 Class: New feature Version: 1.6.0i1 Mattermost is an Open Source alternative to Slack. It provides an equivalent Web Hook API to Slack and thus can be used in the same way as the current Slack notification plugin works.