Switch to managing cronjobs with the Ansible cron module

As suggested in #65 (Github issue), this patch switches
cronjob management from using templates to using Ansible's `cron` module.

It also moves the management of the nginx-reload cronjob to `setup_ssl_lets_encrypt.yml`,
which is a more fitting place for it (given that this cronjob is only required when
Let's Encrypt is used).

Pros:
- using a module is more Ansible-ish than templating our own files in
special directories

- more reliable: will fail early (during playbook execution) if `/usr/bin/crontab`
is not available, which is more of a guarantee that cron is working fine
(idea: we should probably install some cron package using the playbook)

Cons:
- invocation schedule is no longer configurable, unless we define individual
variables for everything or do something smart (splitting on ' ', etc.).
Likely not necessary, however.

- requires us to deprecate and clean-up after the old way of managing cronjobs,
because it's not compatible (using the same file as before means appending
additional jobs to it)

roles/matrix-server/tasks/setup/setup_nginx_proxy.yml

@@ -31,6 +31,7 @@
     - "matrix-synapse.conf"
     - "matrix-riot-web.conf"
 
+
 #
 # Tasks related to setting up matrix-nginx-proxy
 #
@@ -57,12 +58,6 @@
     mode: 0644
   when: matrix_nginx_proxy_enabled
 
-- name: Ensure periodic restarting of matrix-nginx-proxy is configured (for SSL renewal)
-  template:
-    src: "{{ role_path }}/templates/cron.d/matrix-nginx-proxy-periodic-restarter.j2"
-    dest: "/etc/cron.d/matrix-nginx-proxy-periodic-restarter"
-    mode: 0600
-  when: "matrix_nginx_proxy_enabled and matrix_ssl_retrieval_method == 'lets-encrypt'"
 
 #
 # Tasks related to getting rid of matrix-nginx-proxy (if it was previously enabled)
@@ -86,9 +81,3 @@
     path: "/etc/systemd/system/matrix-nginx-proxy.service"
     state: absent
   when: "not matrix_nginx_proxy_enabled and matrix_nginx_proxy_service_stat.stat.exists"
-
-- name: Ensure periodic restarting of matrix-nginx-proxy is removed
-  file:
-    path: "/etc/cron.d/matrix-nginx-proxy-periodic-restarter"
-    state: absent
-  when: "not matrix_nginx_proxy_enabled or matrix_ssl_retrieval_method != 'lets-encrypt'"