api: Add diskInfo as part of StatVol and ListVols. (#1349)

It is the bucket and volumes which needs to have this
value rather than the DiskInfo API itself. Eventually
this can be extended to show disk usage per
Buckets/Volumes whenever we have that functionality.

For now since buckets/volumes are thinly provisioned
this is the right approach.

object-datatypes.go

@@ -22,6 +22,8 @@ import "time"
 type BucketInfo struct {
 	Name    string
 	Created time.Time
+	Total   int64
+	Free    int64
 }
 
 // ObjectInfo - object info.