add --keep-until-expiring closer to letsencrypt command in cron

This change adds "--keep-until-expiring" closer to letsencrypt binary because there can be a pipe or another
command at the end.

Here is an example of broken cron (a lot of parameters is omitted):

certbot -d 'example.com' | grep "Congratulations" && (/sbin/service nginx reload) --keep-until-expiring

This change fixes it to:

certbot --keep-until-expiring -d 'example.com' | grep "Congratulations" && (/sbin/service nginx reload)