As suggested in #63 (Github issue), splitting the
playbook's logic into multiple roles will be beneficial for
maintainability.

This patch realizes this split. Still, some components
affect others, so the roles are not really independent of one
another. For example:
- disabling mxisd (`matrix_mxisd_enabled: false`), causes Synapse
and riot-web to reconfigure themselves with other (public)
Identity servers.

- enabling matrix-corporal (`matrix_corporal_enabled: true`) affects
how reverse-proxying (by `matrix-nginx-proxy`) is done, in order to
put matrix-corporal's gateway server in front of Synapse

We may be able to move away from such dependencies in the future,
at the expense of a more complicated manual configuration, but
it's probably not worth sacrificing the convenience we have now.

As part of this work, the way we do "start components" has been
redone now to use a loop, as suggested in #65 (Github issue).
This should make restarting faster and more reliable.

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)

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.