cephadm-adopt: fix mgr placement hosts task

When no `[mgrs]` group is defined in the inventory, mgr daemon are
implicitly collocated with monitors.
This task currently relies on the length of the mgr group in order to
tell cephadm to deploy mgr daemons.
If there's no `[mgrs]` group defined in the inventory, it will ask
cephadm to deploy 0 mgr daemon which doesn't make sense and will throw
an error.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1970313

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>
(cherry picked from commit f9a73149a4)

infrastructure-playbooks/cephadm-adopt.yml

@@ -367,7 +367,7 @@
         CEPHADM_IMAGE: '{{ ceph_docker_registry }}/{{ ceph_docker_image }}:{{ ceph_docker_image_tag }}'
 
 - name: adopt ceph mgr daemons
-  hosts: "{{ mgr_group_name|default('mgrs') }}"
+  hosts: "{{ groups[mgr_group_name] | default(groups[mon_group_name]) }}"
   serial: 1
   become: true
   gather_facts: false
@@ -1172,8 +1172,16 @@
       environment:
         CEPHADM_IMAGE: '{{ ceph_docker_registry }}/{{ ceph_docker_image }}:{{ ceph_docker_image_tag }}'
 
+    - name: set_fact mgr_placement
+      set_fact:
+        mgr_placement_count: "{{ groups.get(mgr_group_name, []) | length if groups.get(mgr_group_name, []) | length > 0 else groups.get(mon_group_name, []) | length }}"
+
+    - name: set_fact mgr_placement_label
+      set_fact:
+        mgr_placement_label: "{{ mgr_group_name if groups.get(mgr_group_name, []) | length > 0 else mon_group_name }}"
+
     - name: update the placement of manager hosts
-      command: "{{ cephadm_cmd }} shell --fsid {{ fsid }} -- ceph --cluster {{ cluster }} orch apply mgr --placement='{{ groups.get(mgr_group_name, []) | length }} label:{{ mgr_group_name }}'"
+      command: "{{ cephadm_cmd }} shell --fsid {{ fsid }} -- ceph --cluster {{ cluster }} orch apply mgr --placement='{{ mgr_placement_count }} label:{{ mgr_placement_label }}'"
       changed_when: false
       environment:
         CEPHADM_IMAGE: '{{ ceph_docker_registry }}/{{ ceph_docker_image }}:{{ ceph_docker_image_tag }}'