Make roles more independent of one another

With this change, the following roles are now only dependent
on the minimal `matrix-base` role:
- `matrix-corporal`
- `matrix-coturn`
- `matrix-mailer`
- `matrix-mxisd`
- `matrix-postgres`
- `matrix-riot-web`
- `matrix-synapse`

The `matrix-nginx-proxy` role still does too much and remains
dependent on the others.

Wiring up the various (now-independent) roles happens
via a glue variables file (`group_vars/matrix-servers`).
It's triggered for all hosts in the `matrix-servers` group.

According to Ansible's rules of priority, we have the following
chain of inclusion/overriding now:
- role defaults (mostly empty or good for independent usage)
- playbook glue variables (`group_vars/matrix-servers`)
- inventory host variables (`inventory/host_vars/matrix.<your-domain>`)

All roles default to enabling their main component
(e.g. `matrix_mxisd_enabled: true`, `matrix_riot_web_enabled: true`).
Reasoning: if a role is included in a playbook (especially separately,
in another playbook), it should "work" by default.

Our playbook disables some of those if they are not generally useful
(e.g. `matrix_corporal_enabled: false`).

roles/matrix-riot-web/tasks/main.yml

@@ -2,6 +2,12 @@
   tags:
     - always
 
+- import_tasks: "{{ role_path }}/tasks/validate_config.yml"
+  when: "run_setup and matrix_riot_web_enabled"
+  tags:
+    - setup-all
+    - setup-riot-web
+
 - import_tasks: "{{ role_path }}/tasks/setup_riot_web.yml"
   when: run_setup
   tags:

roles/matrix-riot-web/tasks/validate_config.yml

@@ -0,0 +1,9 @@
+---
+
+- name: Fail if required riot-web settings not defined
+  fail:
+    msg: >
+      You need to define a required configuration setting (`{{ item }}`) for using riot-web.
+  when: "vars[item] == ''"
+  with_items:
+    - "matrix_riot_web_default_hs_url"