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-corporal/templates/config.json.j2

@@ -1,9 +1,9 @@
 {
 	"Matrix": {
-		"HomeserverDomainName": "{{ hostname_identity }}",
-		"HomeserverApiEndpoint": "http://matrix-synapse:8008",
-		"AuthSharedSecret": "{{ matrix_synapse_ext_password_provider_shared_secret_auth_shared_secret }}",
-		"RegistrationSharedSecret": "{{ matrix_synapse_registration_shared_secret }}",
+		"HomeserverDomainName": "{{ matrix_corporal_matrix_homeserver_domain_name }}",
+		"HomeserverApiEndpoint": "{{ matrix_corporal_matrix_homeserver_api_endpoint }}",
+		"AuthSharedSecret": "{{ matrix_corporal_matrix_auth_shared_secret }}",
+		"RegistrationSharedSecret": "{{ matrix_corporal_matrix_registration_shared_secret }}",
 		"TimeoutMilliseconds": {{ matrix_corporal_matrix_timeout_milliseconds }}
 	},