This change is provoked by a few different things:

- #54 (Github Pull Request), which rightfully says that we need a
way to support ALL mxisd configuration options easily

- the upcoming mxisd 1.3.0 release, which drops support for
property-style configuration (dot-notation), forcing us to
redo the way we generate the configuration file

With this, mxisd is much more easily configurable now
and much more easily maintaneable by us in the future
(no need to introduce additional playbook variables and logic).

fixes #62

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`).