Explicitly choose our allowed SSH ciphers

The symmetric portion of the SSH Transport Protocol (as described in RFC
4253 [1]) has security weaknesses that allowed recovery of up to 32 bits of
plaintext from a block of ciphertext that was encrypted with the Cipher
Block Chaining (CBD) method. New Counter mode algorithms (as described in
RFC4344 [2]) were designed that are not vulnerable to these types of
attacks and these algorithms are now recommended for standard use.

[1] https://tools.ietf.org/html/rfc4253
[2] https://tools.ietf.org/html/rfc4344