Define variable for additional printer admin group

Add a new variable `printer_admin_group` to specify additional CUPS admin group.
When set to `role-teacher`, teachers will have the ability to add and remove printers.

roles/lmn_printer/tasks/main.yml

@@ -34,10 +34,10 @@
 - name: Allow group role-teacher to manage printers
   ansible.builtin.lineinfile:
     dest: /etc/cups/cups-files.conf
-    line: 'SystemGroup root lpadmin role-teacher'
+    line: "SystemGroup root lpadmin {{ printer_admin_group }}"
     regexp: '^SystemGroup'
     state: present
-  when: "'teacherlaptop' in group_names"
+  when: printer_admin_group | length
 
 - name: Disable cups-browsed
   ansible.builtin.systemd: