crypto/ssh: add support for aes128-cbc cipher.

The aes128cbc cipher is commented out in cipher.go on purpose, anyone wants to
use the cipher needs to uncomment line 119 in cipher.go

Fixes #4274.

Change-Id: I4bbc88ab884bda821c5f155dcf495bb7235c8605
Reviewed-on: https://go-review.googlesource.com/8396
Reviewed-by: Adam Langley <agl@golang.org>

ssh/test/session_test.go

@@ -280,6 +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
+	// fallback to the next available as per line 292.
+	cipherOrder = append(cipherOrder, "aes128-cbc")
 
 	for _, ciph := range cipherOrder {
 		server := newServer(t)