Since commit b9753635 the task 'Ensure docker-ce is installed (Debian)' fails with an error on CentOS although it should not even run on this OS.

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).

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.

fixes #62

It looks like SELinux can be left running without any (so far) negative
effects on our Matrix services.

There's no need to use `:z` or `:Z` options when mounting volumes either.
This means that files we create are labeled with a default context
(which may not be ideal if we only want them used from containers),
but it's compatible and doesn't cause issues.

Relabelling files is probably something we wish to stay away from,
especially for things like the media store, which contains lots of
files and is possibly on a fuse-mounted (S3/goofys) filesystem.

The old way is deprecated and would stop working
after Ansible 2.11.

Improves performance of media store operations.

Let's let the admin set them as they wish.
We don't care what they are anyway.

If other things run on the same server,
it's also better not to hijack these for our
own purposes, especially when we don't need to.

The timedatectl call also seems to fail on Ubuntu 17.04
for some reason (missing timezones information file?).

Some CentOS 7 hosts may not have firewalld installed.
We shouldn't expect it to be, but should ensure by ourselves that it is.

Docker likes to mess around with iptables forwarding rules,
so it ought to start after firewalld.