* appservice: add and use matrix_homeserver_* vars

* appservice: use the new vars

* Apply suggestions from code review

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

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

One that doesn't trip up ansible-lint, causing `load-failure` errors.

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1939

Based on mautrix-whatsapp, as that's what the bridge software is based on.