Fixes #330 (Github Issue).

Continuation of #234 (Github Pull Request).

I had unintentionally updated the documentation for the feature,
saying the page is available at `https://matrix.DOMAIN/nginx_status`.

Looks like it wasn't the case, going against my expectations.

I'm correcting this with this patch.
The status page is being made available on both HTTP and HTTPS.
Serving over HTTP is likely necessary for services like
Longview
(https://www.linode.com/docs/platform/longview/longview-app-for-nginx/)

We recently had someone in the support room who set it to `false`
and the playbook ran without any issues.

This currently seems to yield the same result as 'none', but it's
better to avoid such behavior.

We do use some `:latest` images by default for the following services:
- matrix-dimension
- Goofys (in the matrix-synapse role)
- matrix-bridge-appservice-irc
- matrix-bridge-appservice-discord
- matrix-bridge-mautrix-facebook
- matrix-bridge-mautrix-whatsapp

It's terribly unfortunate that those software projects don't release
anything other than `:latest`, but that's how it is for now.

Updating that software requires that users manually do `docker pull`
on the server. The playbook didn't force-repull images that it already
had.

With this patch, it starts doing so. Any image tagged `:latest` will be
force re-pulled by the playbook every time it's executed.

It should be noted that even though we ask the `docker_image` module to
force-pull, it only reports "changed" when it actually pulls something
new. This is nice, because it lets people know exactly when something
gets updated, as opposed to giving the indication that it's always
updating the images (even though it isn't).

Fixes #191 (Github Issue).

Fixes #171 (Github Issue).

While using certbot means we'll have both files retrieved,
it's actually the fullchain.pem file that we use in nginx configuration.

Using that one for the check makes more sense.

Reasoning is the same as for matrix-org/synapse#5023.

For us, the journal used to contain `docker` for all services, which
is not very helpful when looking at them all together (`journalctl -f`).

Use Jinja2 lstrip_blocks option in templates to ensure consistent
indentation in generated files.

As discussed in #151 (Github Pull Request), it's
a good idea to not selectively apply casting, but to do it in all
cases involving arithmetic operations.

Previously, we'd show an error like this:

{"changed": false, "item": null, "msg": "Detected an undefined required variable"}

.. which didn't mention the variable name
(`matrix_ssl_lets_encrypt_support_email`).

Use an int conversion in the computation of the value of
matrix_nginx_proxy_tmp_directory_size_mb, to have the integer value
multiplied by 50 instead of having the string repeated 50 times.

It doesn't hurt to attempt renewal more frequently, as it only does
real work if it's actually necessary.

Reloading, we postpone some more, because certbot adds some random delay
(between 1 and 8 * 60 seconds) when renewing. We want to ensure
we reload at least 8 minutes later, which wasn't the case.

To make it even safer (in case future certbot versions use a longer
delay), we reload a whole hour later. We're in no rush to start using
the new certificates anyway, especially given that we attempt renewal
often.

Somewhat fixes #146 (Github Issue)

add doc

In most cases, there's not really a need to touch the system
firewall, as Docker manages iptables by itself
(see https://docs.docker.com/network/iptables/).

All ports exposed by Docker containers are automatically whitelisted
in iptables and wired to the correct container.

This made installing firewalld and whitelisting ports pointless,
as far as this playbook's services are concerned.

People that wish to install firewalld (for other reasons), can do so
manually from now on.

This is inspired by and fixes #97 (Github Issue).

By default, `--tags=self-check` no longer validates certificates
when `matrix_ssl_retrieval_method` is set to `self-signed`.

Besides this default, people can also enable/disable validation using the
individual role variables manually.

Fixes #124 (Github Issue)

Currently the nginx reload cron fails on Debian 9 because the path to
systemctl is /bin/systemctl rather than /usr/bin/systemctl.

CentOS 7 places systemctl in both /bin and /usr/bin, so we can just use
/bin/systemctl as the full path.