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-base/tasks/sanity_check.yml

@@ -15,16 +15,6 @@
     msg: "{{ matrix_ansible_outdated_fail_msg }}"
   when: "ansible_version.major == 2 and ansible_version.minor == 5 and ansible_version.revision < 2"
 
-- name: Fail if Macaroon key is missing
-  fail:
-    msg: "You need to set a secret in the matrix_synapse_macaroon_secret_key variable"
-  when: "matrix_synapse_macaroon_secret_key == ''"
-
-- name: Fail if Coturn Auth secret is missing
-  fail:
-    msg: "You need to set a secret in the matrix_coturn_turn_static_auth_secret variable"
-  when: "matrix_coturn_turn_static_auth_secret == ''"
-
 # This sanity check is only used to detect uppercase when people override these specific variables.
 #
 # If people set `host_specific_hostname_identity` without overriding other variables (the general use-case),