Add Mattermost support

What does this Merge Request do?

Adding mattermost to the procfile. I've had a couple of chat-ops things to work on recently and integrating with mattermost is the easiest way work on those in isolation. With the new changes to GDK my procfile changes (adding mattermost) seem to get removed every time I gdk update, so I'm hoping we can add this in.

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.