etcdctl: fix migrate in outputing client.Node to json

Using printf will try to parse the string and replace special
characters. In migrate code, we want to just output the raw
json string of client.Node.
For example,
    Printf("%\\") => %!\(MISSING)
    Print("%\\") => %\
Thus, we should use print instead.

etcdctl/ctlv3/command/migrate_command.go

@@ -220,7 +220,7 @@ func writeKeys(w io.Writer, n *store.NodeExtern) uint64 {
 	if err != nil {
 		ExitWithError(ExitError, err)
 	}
-	fmt.Fprintf(w, string(b))
+	fmt.Fprint(w, string(b))
 	for _, nn := range nodes {
 		max := writeKeys(w, nn)
 		if max > maxIndex {