Add metadata field for backup info
Add metadata field for backup info

Signed-off-by: Dinesh Israni <82c1a936c538eff8920add13416cb0e44e21b2ec@portworx.com>

libopenstorage/openstorage,pault84/openstorage,libopenstorage/openstorage,pault84/openstorage,pault84/openstorage,libopenstorage/openstorage