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/common.go

@@ -206,6 +206,14 @@ func (c *Config) SetDefaults() {
 	if c.Ciphers == nil {
 		c.Ciphers = supportedCiphers
 	}
+	var ciphers []string
+	for _, c := range c.Ciphers {
+		if cipherModes[c] != nil {
+			// reject the cipher if we have no cipherModes definition
+			ciphers = append(ciphers, c)
+		}
+	}
+	c.Ciphers = ciphers
 
 	if c.KeyExchanges == nil {
 		c.KeyExchanges = supportedKexAlgos