* 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>

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1996

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1992

Regression since cf6e38a586

Related to:

- https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1992
- https://github.com/kelaresg/go-skype-bridge/pull/17

Not doing {% if matrix_admin %} checks in the YAML also fixes some issues
with indentation being incorrect sometimes.

This should be backward compatible, except for mautrix-signal's case
where `matrix_mautrix_signal_bridge_permissions` previously existed
as a string, not a dictionary. `tasks/validate_config.yml` will catch
the problem an even provide a quick fix.

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.