Allow the matrix user username and groupname to be configured separately

No migration steps should be required.

roles/matrix-synapse/tasks/setup_synapse.yml

@@ -6,7 +6,7 @@
     state: directory
     mode: 0750
     owner: "{{ matrix_user_username }}"
-    group: "{{ matrix_user_username }}"
+    group: "{{ matrix_user_groupname }}"
   with_items:
     - { path: "{{ matrix_synapse_config_dir_path }}", when: true }
     - { path: "{{ matrix_synapse_run_path }}", when: true }