We used to pass the shared secret for double-puppeting via a
`login_shared_secret` parameter, which doesn't seem to exist anymore.
The proper way to do it is via `login_shared_secret_map`.

The comments for `login_shared_secret_map` seem to indicate that it's
only usable with the shared-secret-auth password provider.

However, this bridge is based on mautrix-python (`>=0.20.5,<0.21`) as
per its `requirements.txt`
Support for double-puppeting via arbitrary access tokens landed in
mautrix-python 0.20.1 (6f25b62e80/CHANGELOG.md (L44-L53),)
so it should be possible to use appservice double-puppet.
Related to af04ca1238

A bunch of other parameters seem to have moved around as well.

This patch introduces some new Ansible variables for controlling
additional settings related to encryption, etc.

This is an attempt at optimizing service startup.

The effect is most pronounced when many services are restarted one by one.
The systemd service manager role sometimes does this - for example when `just install-service synapse` runs.
In such cases, a 5-second delay for each Synapse worker service
(or other bridge/bot service that waits on the homeserver) quickly adds up to a lot.

When services are all stopped fully and then started, the effect is not so pronounced, because
`matrix-synapse.service` starts first and pulls all worker services (defined as `Wants=` for it).
Later on, when the systemd service manager role "starts" these worker services, they're started already.
Even if they had a 5-second wait each, it would have happened in parallel.

`--mount` is safer, as `-v` has the side-effect of creating the "source"
destination as a directory if it doesn't exist yet.
We don't need such magic.

This paves the way for installing other roles into `roles/galaxy` using `ansible-galaxy`,
similar to how it's done in:

- https://github.com/spantaleev/gitea-docker-ansible-deploy
- https://github.com/spantaleev/nextcloud-docker-ansible-deploy

In the near future, we'll be removing a lot of the shared role code from here
and using upstream roles for it. Some of the core `matrix-*` roles have
already been extracted out into other reusable roles:

- https://github.com/devture/com.devture.ansible.role.postgres
- https://github.com/devture/com.devture.ansible.role.systemd_docker_base
- https://github.com/devture/com.devture.ansible.role.timesync
- https://github.com/devture/com.devture.ansible.role.vars_preserver
- https://github.com/devture/com.devture.ansible.role.playbook_runtime_messages
- https://github.com/devture/com.devture.ansible.role.playbook_help

We just need to migrate to those.