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-postgres/tasks/validate_config.yml

@@ -0,0 +1,22 @@
+---
+
+- name: (Deprecation) Warn about matrix_postgres_use_external usage
+  fail:
+    msg: >
+      The `matrix_postgres_use_external` variable defined in your configuration is not used by this playbook anymore!
+      You'll need to adapt to the new way of using an external Postgres server.
+      It's a combination of `matrix_postgres_enabled: false` and specifying Postgres connection
+      details in a few `matrix_synapse_database_` variables.
+      See the "Using an external PostgreSQL server (optional)" documentation page.
+  when: "'matrix_postgres_use_external' in vars"
+
+- name: Fail if required Postgres settings not defined
+  fail:
+    msg: >
+      You need to define a required configuration setting (`{{ item }}`) for using mxisd.
+  when: "vars[item] == ''"
+  with_items:
+    - "matrix_postgres_connection_hostname"
+    - "matrix_postgres_connection_username"
+    - "matrix_postgres_connection_password"
+    - "matrix_postgres_db_name"