Switch Docker image to official one

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)

roles/matrix-server/tasks/main.yml

@@ -28,10 +28,16 @@
     - setup-main
     - setup-goofys
 
+- include: tasks/setup_coturn.yml
+  tags:
+    - setup-main
+    - setup-coturn
+
 - include: tasks/setup_synapse.yml
   tags:
     - setup-main
     - setup-synapse
+    - setup-coturn
 
 - include: tasks/setup_riot_web.yml
   tags: