Support Riot/Matrix

[La0]

Because we are using it ! 🎉

Taskboot should be able to auto-magically send Slack Riot notifications when a deployment action is done (docker push, deploy on S3, Taskcluster artifact or Heroku).

A shared optional argument --notify (with default to env variable NOTIFY) could be used here.
The secret will need to have a riot configuration such as:

riot:
  server: chat.mozilla.org
  token: secretXXX
  channel: my-team-channel