Disallow DES for SSL connections

Older OpenSSL versions include DES-CBC3-* ciphers when specifying the
HIGH group of ciphers. Removing potentially weak ciphers from the list
of allowed ciphers ensures only strong ciphers are considered for SSL
connections.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>