Attempt at fixing #268 (Github Issue)

Fixes #255 (Github Issue).

Continuation of #246 (Github Pull Request)

This only gets triggered if:
- the Synapse role is used standalone and the default values are used
- the whole playbook is used, with `matrix_mxisd_enabled: false`

Also discussed previously in #213 (Github Pull Request).

shared-secret-auth and rest-auth logging is still at `INFO`
intentionally, as user login events seem more important to keep.
Those modules typically don't spam as much.

Related to #223 (Github Pull Request)

ef5e4ad061 intentionally makes us conform to
the logging format suggested by the official Docker image.

Reverting this part, because it's uglier.

This likely should be fixed upstream as well though.

Somewhat related to #213 (Github Pull Request).

We've been moving in the opposite direction for quite a long time.
All services should just leave logging to systemd's journald.

Fixes a regression introduced during the upgrade to
Synapse v1.1.0 (in 2b3865ceea).

Since Synapse v1.1.0 upgraded to Python 3.7
(https://github.com/matrix-org/synapse/pull/5546),
we need to use a different modules directory when mounting
password provider modules.

Fixes a problem introduced by da6edc9cba.

Related to #145 (Github Pull Request).

This is a new feature of Synapse v1.1.0.

Discussed in #145 (Github Pull Request).

Using `|to_json` on a string is expected to correctly wrap it in quotes (e.g. `"4"`).
Wrapping it explicitly in double-quotes results in undesirable double-quoting (`""4""`).

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).

Bridges start matrix-synapse.service as a dependency, but
Synapse is sometimes slow to start, while bridges are quick to
hit it and die (if unavailable).

They'll auto-restart later, but .. this still breaks `--tags=start`,
which doesn't wait long enough for such a restart to happen.

This attempts to slow down bridge startup enough to ensure Synapse
is up and no failures happen at all.

Attempt to fix #192 (Github Issue), potential regression since
70487061f4.

Serializing as JSON/YAML explicitly is much better than relying on
magic (well, Python serialization being valid YAML..).
It seems like Python may prefix strings with `u` sometimes (Python 3?),
which causes Python serialization to not be compatible with YAML.

This doesn't replace all usage of `-v`, but it's a start.

People sometimes troubleshoot by deleting files (especially bridge
config files). Restarting Synapse with a missing registration.yaml file
for a given bridge, causes the `-v
/something/registration.yaml:/something/registration.yaml:ro` option
to force-create `/something/registration.yaml` as a directory.

When a path that's provided to the `-v` option is missing, Docker
auto-creates that path as a directory.
This causes more breakage and confusion later on.

We'd rather fail, instead of magically creating directories.
Using `--mount`, instead of `-v` is the solution to this.

From Docker's documentation:

> When you use --mount with type=bind, the host-path must refer to an existing path on the host.
> The path will not be created for you and the service will fail with an error if the path does not exist.

Fixes #171 (Github Issue).

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`).