* 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.

https://github.com/matrix-org/matrix-hookshot/releases/tag/1.4.0

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

This fixes parameters not being passed to the provisioning api

Hello, this PR should fix Hookshot container not receiving API calls with the right path because nginx proxy was stripping the /v1 from it.

to proxy url params in GET requests

see also #1527