Add support to incremental backups in cinder

Add support to incremental backups in cinder

1. Added a new option to create backup to support incremental backups.
   New CLI option added for "cinder backup": --incremental or --incr
   By default, it is full backup.
   Add the following to API: 'incremental': True or False
   (False is the default)
2. swift.py creates sha256file for every backup to calculate deltas

DocImpact
APIImpact
Co-Authored-By: Xing Yang <xing.yang@emc.com>
Implements: blueprint incremental-backup
Change-Id: I98f3765f7afa47bda6a8c7004f809c3cc4d1bd4b