Send usage notifications on major stack events

These notifications can then be used for usage/billing/alarming purposes.

Some of the base fields are based on the existing nova fields:
https://wiki.openstack.org/wiki/SystemUsageData

This patch just has the minimum fields needed we can add more as users
ask.

We can also add more notifications as we need:
- automated actions (autoscaling, restart)
- time based .exists notifications

implements blueprint send-notification
Change-Id: I63ad5402037799842643eb642af15b54ef8d7483
Merge "Send usage notifications on major stack events"