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.

docs/configuring-playbook-mxisd.md

@@ -29,7 +29,7 @@ You can refer to the [mxisd website](https://github.com/kamax-io/mxisd) for more
 
 To use a more custom configuration, you can define a `matrix_mxisd_configuration_extension_yaml` string variable
 and put your configuration in it.
-To learn more about how to do this, refer to the information about `matrix_mxisd_configuration_extension_yaml` in the [default variables file](../roles/matrix-server/defaults/main.yml).
+To learn more about how to do this, refer to the information about `matrix_mxisd_configuration_extension_yaml` in the [default variables file](../roles/matrix-mxisd/defaults/main.yml) of the mxisd component.
 
 
 ## Troubleshooting

docs/configuring-playbook.md

@@ -8,7 +8,7 @@ You can follow these steps:
 
 - copy the sample configuration file (`cp examples/host-vars.yml inventory/host_vars/matrix.<your-domain>/vars.yml`)
 
-- edit the configuration file (`inventory/host_vars/matrix.<your-domain>/vars.yml`) to your liking. You may also take a look at `roles/matrix-server/defaults/main.yml` and see if there's something you'd like to copy over and override in your `vars.yml` configuration file.
+- edit the configuration file (`inventory/host_vars/matrix.<your-domain>/vars.yml`) to your liking. You may also take a look at the various `roles/ROLE_NAME_HERE/defaults/main.yml` files and see if there's something you'd like to copy over and override in your `vars.yml` configuration file.
 
 - copy the sample inventory hosts file (`cp examples/hosts inventory/hosts`)