Support Volume Backup Quota

Since quota take volumes, snapshots and gigabytes into account, it also
needs to take backup into account.
1. Backup create API is not admin interface, users of projects could
create any number of backups.
2. If some evil users create many more big backups to exhaust the free
space of backup storage-backend, it would cause cinder-backup in the state
of rejecting service.
Based on these two points, I think it is necessary to support volume
backup quota.

blueprint support-volume-backup-quota

Change-Id: Idd24ad2e8a5507bf18e90bd5fad59a4af1c158c6