Client support for export and import backup service metadata

Add client code to a new admin API for backup-export and
backup-import allowing an admin to copy the backup metadata
details.

client code for https://review.openstack.org/#/c/69351/

blueprint cinder-backup-recover-api
DocImpact new admin API calls backup-import and backup-export

Change-Id: Id306fc9cf562a87bb6a36eae692852eefc592b74
Merge "Client support for export and import backup service metadata"