Added a new option to "cinder backup-create" to support incremental backups.
Specify --incremental to create an incremental backup. By default, it will
be a full backup.
Co-Authored-By: Xing Yang <xing.yang@emc.com>
Implements: blueprint incremental-backup
Change-Id: Id11ecd2982ea838181b4adaa5a742fc65f864acb