Support import/export snapshots in cinder

Currently cinder support import/export volume function. Import/export
snapshots function could be an complement for the function of
import/export volume.

Functions:
1.It could provide the ability to import volumes' snapshot from one cinder
to another cinder,
and import "non" openstack snapshots already on a back-end device into
OpenStack/cinder.
2. Additionally, export snapshots works the same way as export volumes.

Benefits:
When import volumes that have snapshots, we need to import snapshots.
1. First we could use the import snapshots as volume templates to create
volumes.
2. Import snapshots function could provide an effect mean to manage the
import
volumes.
    For example, we could not delete the import volume that has snapshots.
    By using import snapshots function to import snapshots, we could
    first delete the import snapshots, and then delete the import volumes.

blueprint support-import-export-snapshots

Change-Id: I27b29925343d9c634b82cce1f5e28bc1a609bc98
Merge "Support import/export snapshots in cinder"