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)

CHANGELOG.md

@@ -1,3 +1,15 @@
+# 2019-01-08
+
+## (BC Break) Cronjob schedule no longer configurable
+
+Due to the way we manage cronjobs now, you can no longer configure the schedule they're invoked at.
+
+If you were previously using `matrix_ssl_lets_encrypt_renew_cron_time_definition` or `matrix_nginx_proxy_reload_cron_time_definition`
+to set a custom schedule, you should note that these variables don't affect anything anymore.
+
+If you miss this functionality, please [open an Issue](https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/new) and let us know about your use case!
+
+
 # 2018-12-23
 
 ## (BC Break) More SSL certificate retrieval methods