storage: Implement GetDiskID request in REST server side (#9720)

GetDiskID() in storage rest client does not really issue a REST request
to the remote disk, but returns an in-memory value instead.

However, GetDiskID() should return an error when format.json is not
found or for other similar issues (unmounted disks, etc..)

GetDiskID() is only called when formatting disks and getting storage
informatio, hence this commit should not have a performance degradation.

cmd/storage-interface.go

@@ -31,7 +31,7 @@ type StorageAPI interface {
 	IsLocal() bool
 	Hostname() string // Returns host name if remote host.
 	Close() error
-	GetDiskID() (string, error)
+	GetDiskID() (string, error) // Could be expensive
 	SetDiskID(id string)
 
 	DiskInfo() (info DiskInfo, err error)