Fix the magnum description

refer to https://github.com/openstack/magnum/blob/master/devstack/lib/magnum#L113

Change-Id: I5207a7ba708daa17f65a2eb61282a265342f3eee

ansible/roles/magnum/tasks/register.yml

@@ -5,7 +5,7 @@
     module_args:
       service_name: "magnum"
       service_type: "container-infra"
-      description: "Openstack Container Service"
+      description: "Container Infrastructure Management Service"
       endpoint_region: "{{ openstack_region_name }}"
       url: "{{ item.url }}"
       interface: "{{ item.interface }}"