Seems like we unintentionally removed the mounting of certificates
(the `/matrix-config` mount) as part of splitting the playbook into
roles in 51312b8250.

It appears that those certificates weren't necessary for coturn to
funciton though, so we might just get rid of the configuration as well.

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.

Coturn reports it as an option that conflicts with `use-auth-secret`.
Some reasoning is here: fa523e8d09

Switching from from avhost/docker-matrix (silviof/docker-matrix)
to matrixdotorg/synapse.

The avhost/docker-matrix (silviof/docker-matrix) image used to bundle
in the coturn STUN/TURN server, so as part of the move,
we're separating this to a separately-ran service
(matrix-coturn.service, powered by instrumentisto/coturn-docker-image)