Allow default room version to be configured

roles/matrix-synapse/defaults/main.yml

@@ -286,3 +286,5 @@ matrix_synapse_room_list_publication_rules:
     alias: "*"
     room_id: "*"
     action: allow
+
+matrix_synapse_default_room_version: "4"

roles/matrix-synapse/templates/synapse/homeserver.yaml.j2

@@ -82,7 +82,7 @@ use_presence: {{ matrix_synapse_use_presence|to_json }}
 # For example, for room version 1, default_room_version should be set
 # to "1".
 #
-#default_room_version: "4"
+default_room_version: "{{ matrix_synapse_default_room_version|to_json }}"
 
 # The GC threshold parameters to pass to `gc.set_threshold`, if defined
 #