Split playbook into multiple roles

As suggested in #63 (Github issue), splitting the
playbook's logic into multiple roles will be beneficial for
maintainability.

This patch realizes this split. Still, some components
affect others, so the roles are not really independent of one
another. For example:
- disabling mxisd (`matrix_mxisd_enabled: false`), causes Synapse
and riot-web to reconfigure themselves with other (public)
Identity servers.

- enabling matrix-corporal (`matrix_corporal_enabled: true`) affects
how reverse-proxying (by `matrix-nginx-proxy`) is done, in order to
put matrix-corporal's gateway server in front of Synapse

We may be able to move away from such dependencies in the future,
at the expense of a more complicated manual configuration, but
it's probably not worth sacrificing the convenience we have now.

As part of this work, the way we do "start components" has been
redone now to use a loop, as suggested in #65 (Github issue).
This should make restarting faster and more reliable.

CHANGELOG.md

@@ -1,3 +1,15 @@
+# 2019-01-xx
+
+## Splitting the playbook into multiple roles
+
+For better maintainability, the playbook logic (which all used to reside in a single `matrix-server` role)
+has been split out into a number of different roles: `matrix-synapse`, `matrix-postgres`, `matrix-riot-web`, `matrix-mxisd`, etc. (see the `roles/` directory).
+
+To keep the filesystem more consistent with this separation, the **Postgres data had to be relocated**.
+
+The default value of `matrix_postgres_data_path` was changed from `/matrix/postgres` to `/matrix/postgres/data`. The `/matrix/postgres` directory is what we consider a base path now (new variable `matrix_postgres_base_path`). **Your Postgres data files will automatically be relocated by the playbook** (`/matrix/postgres/*` -> `/matrix/postgres/data/`) when you run with `--tags=setup-all` (or `--tags=setup-postgres`). While this shouldn't cause data-loss, **it's better if you do a Postgres backup just in case**. You'd need to restart all services after this migration (`--tags=start`).
+
+
 # 2019-01-11
 
 ## (BC Break) mxisd configuration changes
@@ -30,7 +42,7 @@ The following variables are no longer supported by this playbook:
 - `matrix_mxisd_template_config`
 
 You are encouraged to use the `matrix_mxisd_configuration_extension_yaml` variable to define your own mxisd configuration additions and overrides.
-Refer to the [default variables file](roles/matrix-server/defaults/main.yml) for more information.
+Refer to the [default variables file](roles/matrix-mxisd/defaults/main.yml) for more information.
 
 This new way of configuring mxisd is beneficial because:
 
@@ -92,7 +104,7 @@ Based on feedback from others, running Synapse on Python 3 is supposed to decrea
 ## Riot homepage customization
 
 You can now customize some parts of the Riot homepage (or even completely replace it with your own custom page).
-See the `matrix_riot_web_homepage_` variables in `roles/matrix-server/defaults/main.yml`.
+See the `matrix_riot_web_homepage_` variables in `roles/matrix-riot-web/defaults/main.yml`.
 
 
 # 2018-12-04