Validate assertion that the init is systemd.

The debian_server role has some systemd specific parts, validate the
assertion that indeed systemd is being used.

roles/debian_server/tasks/main.yml

@@ -4,6 +4,7 @@
     that:
       - ansible_distribution == "Debian"
       - ansible_distribution_major_version|int >= 10
+      - ansible_service_mgr == "systemd"
 
 - name: Disable cgroup2 for Docker
   lineinfile: