Add support for ciphers and macs override in ssh

By default, the SSH server allows connections using weak cipher and MAC algorithms. This commit adds support to override the default Ciphers and MACs options in SSH server.

For example:
set service ssh ciphers aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,arcfour
set service ssh macs hmac-sha1,hmac-ripemd160