This:

- brings consistency - no more mixing `_name_prefix` and `_registry_prefix`
- adds extensibility - a future patch will allow reconfiguring all registry prefixes for all roles in the playbook

We still have `_docker_` vs `_container_` inconsistencies.
These may be worked on later.

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

Based on https://github.com/matrix-org/rageshake

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

Continuation of 9f2eff2ac7

Provoked by 7749048bf8
(https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/3353)

Related to:

- https://github.com/matrix-org/rageshake/issues/69
- https://github.com/matrix-org/rageshake/pull/70

This makes it consistent with the rest of the playbook:
- there's a default config which has various variables controlling
  settings
- there's also an `_extension_yaml` variable, which lets you override it

We disable it via `group_vars/matrix_servers`, as we do for all other
roles.