allow cron renew arguments to be configurable

Default implementation provides a backward compatible solution
and fixes the issue where the "--quiet" argument doesn't exist
anymore in Ubuntu 16.04's default installation of letsencrypt.

tasks/renew-cron.yml

@@ -2,7 +2,7 @@
 - name: Add cron job for certbot renewal (if configured).
   cron:
     name: Certbot automatic renewal.
-    job: "{{ certbot_script }} renew --quiet --no-self-upgrade"
+    job: "{{ certbot_script }} renew {{ certbot_auto_renew_args }}"
     minute: "{{ certbot_auto_renew_minute }}"
     hour: "{{ certbot_auto_renew_hour }}"
     user: "{{ certbot_auto_renew_user }}"

vars/Ubuntu-16.04.yml

@@ -1 +1,2 @@
 certbot_package: letsencrypt
+certbot_auto_renew_args: "--no-self-upgrade"

vars/default.yml

@@ -1,2 +1,3 @@
 ---
 certbot_package: certbot
+certbot_auto_renew_args: "--quiet --no-self-upgrade"