* 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>

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

Details here: https://docs.ansible.com/ansible/devel/porting_guides/porting_guide_6.html#id36

Basically:

```yaml
- name: Prior to 2.13
  debug:
    msg: '[1] + {{ [2] }}'

- name: 2.13 and forward
  debug:
    msg: '{{ [1] + [2] }}'
```

Interestingly, we had been using the new/safe syntax in lofs of places.

We were using the broken one in many others though. Hopefully all
instances were fixed by this patch.

`matrix_nginx_proxy_enabled` is usually set to false by users, this is when
the warning should be displayed.

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1108

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1070

Related discussion here: 1ab507349c (commitcomment-51108407)

We'll be adding `.timer` units later on, so it's good to be
more explicit.

Regression since 4e8543ce21

Until now, if `--tags=setup-synapse` was used, bridge tasks would not
run and bridges would fail to register with the `matrix-synapse` role.
This means that Synapse's configuration would be generated with an empty
list of appservices (`app_service_config_files: []`).

.. and then bridges would fail, because Synapse would not be aware of
there being any bridges.

From now on, bridges always run their init tasks and always register
with Synapse.

For the Telegram bridge, the same applies to registering with
matrix-nginx-proxy. Previously, running `--tags=setup-nginx-proxy` would
get rid of the Telegram endpoint configuration for the same reason.
Not anymore.

The goal is to move each bridge into its own separate role.
This commit starts off the work on this with 2 bridges:
- mautrix-telegram
- mautrix-whatsapp

Each bridge's role (including these 2) is meant to:

- depend only on the matrix-base role

- integrate nicely with the matrix-synapse role (if available)

- integrate nicely with the matrix-nginx-proxy role (if available and if
required). mautrix-telegram bridge benefits from integrating with
it.

- not break if matrix-synapse or matrix-nginx-proxy are not used at all

This has been provoked by #174 (Github Issue).

As suggested in #63 (Github issue), splitting the
playbook's logic into multiple roles will be beneficial for
maintainability.

This patch realizes this split. Still, some components
affect others, so the roles are not really independent of one
another. For example:
- disabling mxisd (`matrix_mxisd_enabled: false`), causes Synapse
and riot-web to reconfigure themselves with other (public)
Identity servers.

- enabling matrix-corporal (`matrix_corporal_enabled: true`) affects
how reverse-proxying (by `matrix-nginx-proxy`) is done, in order to
put matrix-corporal's gateway server in front of Synapse

We may be able to move away from such dependencies in the future,
at the expense of a more complicated manual configuration, but
it's probably not worth sacrificing the convenience we have now.

As part of this work, the way we do "start components" has been
redone now to use a loop, as suggested in #65 (Github issue).
This should make restarting faster and more reliable.