x/crypto/ssh: add 3des-cbc as a non-default cipher

3des-cbc is an insecure cipher. As such, you must explictly add it to
Config in order to use it.

Change-Id: Ifd15cde46a9908eefef1c75bae7e97b05767361d
Reviewed-on: https://go-review.googlesource.com/22770
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>

ssh/test/session_test.go

@@ -280,9 +280,9 @@ func TestCiphers(t *testing.T) {
 	var config ssh.Config
 	config.SetDefaults()
 	cipherOrder := config.Ciphers
-	// This cipher will not be tested when commented out in cipher.go it will
+	// These ciphers will not be tested when commented out in cipher.go it will
 	// fallback to the next available as per line 292.
-	cipherOrder = append(cipherOrder, "aes128-cbc")
+	cipherOrder = append(cipherOrder, "aes128-cbc", "3des-cbc")
 
 	for _, ciph := range cipherOrder {
 		server := newServer(t)