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)

The long-deprecated (since Ansible 2.4) use of include is
no more.

We always skip at least one of these tasks, depending on which
SSL retrieval method is enabled, so it could have been confusing why.

Fix for d28bdb3258.

We were only supposed to install openssl when the self-signed
SSL certificate retrieval method is used, not always.

Fix for d28bdb3258

Adds support for managing certificates manually and for
having the playbook generate self-signed certificates for you.

With this, Let's Encrypt usage is no longer required.

Fixes Github issue #50.