https://letsencrypt.org/getting-started/
https://certbot.eff.org/lets-encrypt/centosrhel7-nginx
$ echo "0 0,12 * * * root python -c 'import random; import time; time.sleep(random.random() * 3600)' && certbot renew -q" | sudo tee -a /etc/crontab > /dev/null