set matrix host name to netz39.de

this will appear in all matrix user handles

host-pottwal.yml

@@ -42,6 +42,7 @@
         # RabbitMQ setup can be found in the inventory
     - role: netz39.matrix_synapse
       vars:
+        matrix_host_name: netz39.de
         matrix_data_path: "{{ data_dir }}/matrix/data"
         matrix_media_path: "{{ data_dir }}/matrix/media"
         matrix_postgres_data_path: "{{ data_dir }}/matrix/postgres"