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.

* Created role for synapse-usage-exporter

* Apply suggestions from code review

Co-authored-by: Slavi Pantaleev <slavi@devture.com>

* Renaming docker variables and moving synapse stats config location

* Respect devture_systemd_docker_base_docker_service_name

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Co-authored-by: Slavi Pantaleev <slavi@devture.com>