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>