Improve volume notifications

Volume notifications are currently missing a number of fields that can be retrieved from the volume/get API.

For instance, update.end (or attach.end) notifications are missing the following fields the API returns for a volume (from a stock, recent devstack instance):

bootable, consistencygroup_id, description, encrypted, migration_status, multiattach, source_volid,

In addition, volume_type is an id in notifications versus a name from the API, and os-vol-mig-status-* fields from the API response seem to have no equivalent in notifications.

Snapshot notifications are much smaller; fields missing in snapshot notifications are 'description' and the 'os-extended-snapshot-attributes'.

This might be something that benefits from versioned notifications (https://blueprints.launchpad.net/cinder/+spec/versioned+notification) if changing format is a problem (though this would largely be an additive change).