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: I8687035f4a1bf50766cc913c5adc20a373e8365d
Merge "Support Volume Backup Quota"