Add autoscaling notifications

Send 2 notifications when an auto-scaling event occurs :
- successful resizing : 'autoscaling.start' and 'autoscaling.end'
- failed resizing : 'autoscaling.start' and 'autoscaling.error'

Implements blueprint send-notification-autoscaling

In same time, split out all notification code in its own module :
- stack CRUD : heat/engine/notification/stack.py
- autoscaling events : heat/engine/notification/autoscaling.py

Co-Authored-By: Angus Salkeld <angus@salkeld.id.au>
(from https://review.openstack.org/#/c/54661/)

Change-Id: I23e4478faf58ef23b031a3b0d4087c892dc4e96f
Merge "Add autoscaling notifications"