feat(global_defaults): add root.shell to system schema and normalization

roles/configuration/tasks/users.yml

@@ -11,7 +11,7 @@
 
 - name: Set root shell
   ansible.builtin.command: >-
-    {{ chroot_command }} /usr/sbin/usermod --shell {{ system_cfg.root.shell | default('/bin/bash') }} root
+    {{ chroot_command }} /usr/sbin/usermod --shell {{ system_cfg.root.shell }} root
   register: configuration_root_shell_result
   changed_when: configuration_root_shell_result.rc == 0