Remove Notification in preference of Control for a 0.5.0 release

[russmiles]

I'm proposing that we remove the notification Humio integration option in preference of the more powerful control based approach. Currently this extension offers both, but Controls are now the preferred approach from the Chaos Toolkit's perspective and so retiring the notification-based approach would reduce the code and reduce the complexity of this extension.

The plan would be to keep the 0.4.x releases to maintain, if necessary, both the control and notification approaches, but that the 0.5.0 would remove notifications going forward. This means that the current 0.4.2 release would likely be the last one to include the notification based approach.