remove sha1 key-exchange mechanisms from default
remove sha1 key-exchange mechanisms from default

Move diffie-hellman-group14-sha1 and diffie-hellman-group-exchange-sha1 to 'weak' KEX mechanisms.

References:
* https://stribika.github.io/2015/01/04/secure-secure-shell.html
* https://github.com/TelekomLabs/chef-ssh-hardening/issues/64

Signed-off-by: Dominik Richter <b7323add517e187efe0283b76511b7ed762bfe13@gmail.com>

michaelklishin/chef-ssh-hardening,patcon/chef-ssh-hardening,dev-sec/chef-ssh-hardening,hardening-io/chef-ssh-hardening,michaelklishin/chef-ssh-hardening,IFTTT/chef-ssh-hardening,IFTTT/chef-ssh-hardening,hardening-io/chef-ssh-hardening,dev-sec/chef-ssh-hardening,patcon/chef-ssh-hardening