Don't load systemd parent service for object cache

We use the systemd_service role to load a drop-in for all
services which fall under the 'ceph-immutable-object-store'
banner, but this isn't a service in its own right.

Attempting to load the service on Ubuntu Jammy results in
an error, so we prevent loading it, and leave enabling of the
individual service up to an existing later task.

Change-Id: If9c46d22f42bc3765b217b0fbc736331bf337557

tasks/ceph_immutable_object_cache.yml

@@ -32,6 +32,7 @@
     systemd_services:
       - service_name: "{{ ceph_immutable_object_cache_service_name }}"
         systemd_overrides_only: true
+        load: false
         systemd_overrides:
           Service:
             UMask: "{{ ceph_immutable_object_cache_umask }}"