Add the availability_zone to the volume.usage notifications

The volume.usage notifications are used to enable traffic based billing
on volumes. The availability_zone of the instance is an important piece of
information that can enable more specialized billing of customers.

This makes the volume.usage events consistent with the volume.create.start/end
snapshot.create.start/end, etc. emitted from cinder, which contain the
availability_zone field.

Change-Id: I9b6b03174cd03235d0e6cf01e34a6da13bd9bb70
Merge "Add the availability_zone to the volume.usage notifications"