cephadm-adopt: Alertmanager placement count missing

Regression from #7576. Alertmanager placement count was missing
after migration to ceph_orch_apply module

Signed-off-by: Teoman ONAY <tonay@ibm.com>

infrastructure-playbooks/cephadm-adopt.yml

@@ -1485,6 +1485,7 @@
               service_id: "{{ ansible_facts['hostname'] }}"
               placement:
                 label: "{{ monitoring_group_name }}"
+                count: "{{ groups.get(monitoring_group_name, []) | length }}"
               {% if grafana_server_addr is defined %}
               networks:
               - {{ grafana_server_addr }}