Allow the matrix user username and groupname to be configured separately

No migration steps should be required.

roles/matrix-jitsi/tasks/setup_jitsi_jicofo.yml

@@ -10,7 +10,7 @@
     state: directory
     mode: 0777
     owner: "{{ matrix_user_username }}"
-    group: "{{ matrix_user_username }}"
+    group: "{{ matrix_user_groupname }}"
   with_items:
     - { path: "{{ matrix_jitsi_jicofo_base_path }}", when: true }
     - { path: "{{ matrix_jitsi_jicofo_config_path }}", when: true }