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
(cherry picked from commit 2bb6cdf481)

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 }}"