Export and import backup service metadata

Export and import backup service metadata

Add new admin API for backup-export and backup-import.
The new commands export the backup details (not actual backup) to
a string that can be imported again in another OpenStack cloud or
if the backup database was corrupted.
The code includes a default backup driver implementation.
Backup test code converted to use mock.

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

Change-Id: I564194929962e75c67630e73d8711ee6587706d4