roles/certbot: Run periodically as certbot user

The default systemd unit configuration for *certbot-renew.service* runs
the `certbot renew …` command as root. This can cause permissions
issues, since this Ansible role expects the *certbot* user to be able to
access all configuration, data, and log files. As such, this commit adds
a systemd unit extension for *certbot-renew.service* to run the command
as *certbot*.

roles/certbot/handlers/main.yml

@@ -1,3 +1,5 @@
+- name: reload systemd
+  command: systemctl daemon-reload
 - name: reload httpd
   service:
     name=httpd