cinder backup-create has an parameter --incremental, but this parameter is not used in drivers/ceph.py

[zheng yin]

usage: cinder backup-create [--container <container>] [--name <name>]
                            [--description <description>] [--incremental]
                            [--force] [--snapshot-id <snapshot-id>]
                            <volume>

Creates a volume backup.

Positional arguments:
  <volume> Name or ID of volume to backup.

Optional arguments:
  --container <container>
                        Backup container name. Default=None.
  --name <name> Backup name. Default=None.
  --description <description>
                        Backup description. Default=None.
  --incremental Incremental backup. Default=False.
  --force Allows or disallows backup of a volume when the volume
                        is attached to an instance. If set to True, backs up
                        the volume whether its status is "available" or "in-
                        use". The backup of an "in-use" volume means your data
                        is crash consistent. Default=False.
  --snapshot-id <snapshot-id>
                        ID of snapshot to backup. Default=None

however, --incremental is not used.. Whether it is true or false, ceph attempts incremental backup.