incremental backup improvements for L

At Kilo release we supported incremental backup, but there are still some points need to be improved.
1. From the API perspective, there is no place to show the backup is incremental or not. It's useful to 3rd part billing system, they could have different fee for full and incremental backup.
2. User also don't the know if the incremental backup can be deleted or not, it's important because cinder don't allow to delete this backup since 'Incremental backups exist for this backup'. At now, they must have a try to know it. So if there is a flag
to indicate the backup can't be deleted or not, it will bring more convenient to user and
reduce API call.
3.Enriching the notification system via reporting to Ceilometer, add parent_id to report.