Added instructions on volume backup and export

This patch adds two sections: one for volume backup, and the other for
exporting volume metadata. The feature in focus here is the volume
metadata export, which makes volume backups portable across cloud
services and also recoverable even in the event of cinder database
failure.

Partially implements: https://blueprints.launchpad.net/cinder/+spec/cinder-backup-recover-api
Partially implements: https://blueprints.launchpad.net/cinder/+spec/cinder-backup-volume-metadata-support

Change-Id: I0ffa0c7ab9b9ecd3449e93390099e8d0001c8efd
Merge "Added instructions on volume backup and export"