Since cbee084ac1, this playbook supports Postgres 10.x,
but keeps existing Postgres-9.x installs on 9.x.
This playbook can now also be ran with `--tags=upgrade-postgres`
to make it upgrade from Postgres 9.x to 10.x (or other versions
in the future).
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)
