Add support for force-delete backups

Add support for force-delete backups

1. Add _get and _delete method to BackupAdminController.
2. Add force=False arg to delete method in backup api.
3. If force=True, then allow the deletion of backup, which
can be in any state.
4. For chunkeddriver based backup, check the status of backup
from DB in backup loop, if it has changed to deleting, set the
flag is_backup_canceled and clear the data to avoid leaving orphan
object in the object store.
5. Add the flag: support_force_delete=False in BackupDriver. That
indicates if the backup driver supports force deletion.
It should be set to True if the driver that inherits from BackupDriver
supports the force deletion function.
6. If the backup driver doesn't support this feature, and it will return
405 when calling API of force delete.

DocImpact
APIImpact
Add os-force_delete in request body:
*POST /v2/{tenant_id}/backups/{id}/action
{
  "os-force_delete": {}
}
Implements: blueprint support-force-delete-backup

Change-Id: Ia4d36520d2a092730cd1be56e338349e383f9e6b