We have 2 blockers that prevent us from adding support:

- the Docker CE repository does not publish a `docker-ce` package
in the `stable` channel. It's still in `edge`
(can be worked around by using `edge`, but we'd better not)

- Docker bind propagation has troubles on Docker CE 18.05,
which breaks matrix-synapse.service from starting, as it wants to do
a `:slave` mount. See https://github.com/moby/moby/issues/37032

Since cbee084ac19b6, this playbook supports Postgres 10.x,
but keeps existing Postgres-9.x installs on 9.x.

This playbook can now also be ran with `--tags=upgrade-postgres`
to make it upgrade from Postgres 9.x to 10.x (or other versions
in the future).

Improves performance of media store operations.

Silvio announced that he's no longer maintaining his images,
so we're jumping to AVENTER-UG's fork.

The non-working script is supposed to be fixed
by https://github.com/matrix-org/synapse/pull/2375

To have it work, we'd need an updated Docker image
of `silviof/matrix-riot-docker:latest`, which is not yet available
at the time of this commit.

Still, the previous patched synapse_port_db didn't work well either,
so it's not like we're regressing much by getting rid of it.

By using role defauts, we can have inventory variables
which overide the defaults.