After having multiple people report issues with retrieving
SSL certificates, we've finally discovered the culprit to be
Ansible 2.5.1 (default and latest version on Ubuntu 18.04 LTS).

As silly as it is, certain distributions ("LTS" even) are 13 bugfix
versions of Ansible behind.

From now on, we try to auto-detect buggy Ansible versions and tell the
user. We also provide some tips for how to upgrade Ansible or
run it from inside a Docker container.

My testing shows that Ansible 2.4.0 and 2.4.6 are OK.
All other intermediate 2.4.x versions haven't been tested, but we
trust they're OK too.

From the 2.5.x releases, only 2.5.0 and 2.5.1 seem to be affected.
Ansible 2.5.2 corrects the problem with `include_tasks` + `with_items`.

If uppercase is used, certain tools (like certbot) would cause trouble.
They would retrieve a certificate for the lowercased domain name,
but we'd try to use it from an uppercase-named directory, which will
fail.

Besides certbot, we may experience other trouble too.
(it hasn't been investigated how far the breakage goes).

To fix it all, we lowercase `host_specific_hostname_identity` by default,
which takes care of the general use-case (people only setting that
and relying on us to build the other domain names - `hostname_matrix`
and `hostname_riot`).

For others, who decide to override these other variables directly
(and who may work around us and introduce uppercase there directly),
we also have the sanity-check tool warn if uppercase is detected
in any of the final domains.