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.

Pretty much all variables live in their own `matrix_<whatever>`
prefix now and are grouped closer together in the default
variables file (`roles/matrix-server/defaults/main.yml`).

`--log-driver=none` is used for all Docker containers now.

All these containers are started through systemd anyway and get logged in journald,
so there's no need for Docker to be logging the same thing using the default `json-file` driver.
Doing that was growing `/var/lib/docker/containers/..` infinitely until service/container restart.

As a result of this, things like `docker logs matrix-synapse` won't work anymore.
`journalctl -u matrix-synapse` is how one can see the logs.

Improves performance of media store operations.