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`).

examples/host-vars.yml

@@ -1,5 +1,5 @@
 # This is something which is provided to Let's Encrypt
-# when retrieving the SSL certificates for `<your-domain>`.
+# when retrieving the SSL certificates for domains.
 #
 # In case SSL renewal fails at some point, you'll also get
 # an email notification there.
@@ -11,7 +11,7 @@
 # Example value: someone@example.com
 host_specific_matrix_ssl_lets_encrypt_support_email: YOUR_EMAIL_ADDRESS_HERE
 
-# This is your bare domain name (`<your-domain`).
+# This is your bare domain name (`<your-domain>`).
 #
 # Note: the server specified here is not touched.
 #
@@ -22,7 +22,7 @@ host_specific_matrix_ssl_lets_encrypt_support_email: YOUR_EMAIL_ADDRESS_HERE
 # Example value: example.com
 host_specific_hostname_identity: YOUR_BARE_DOMAIN_NAME_HERE
 
-# A shared secret (between Synapse and Coturn) used for authentication.
+# A shared secret (between Coturn and Synapse) used for authentication.
 # You can put any string here, but generating a strong one is preferred (e.g. `pwgen -s 64 1`).
 matrix_coturn_turn_static_auth_secret: ""