rolling_update: fix pre and post osd upgrade play

when using --limit osds, the play before and after osd upgrade are
skipped because we use `hosts: "{{ mon_group_name | default('mons') }}[0]"`
using `hosts: "{{ osds_group_name | default('osds') }}" with
`delegate_to` to the first monitor addresses this issue.

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>
(cherry picked from commit fc9f87c45f)

infrastructure-playbooks/rolling_update.yml

@@ -423,7 +423,7 @@
 
 
 - name: set osd flags
-  hosts: "{{ mon_group_name | default('mons') }}[0]"
+  hosts: "{{ osd_group_name | default('osds') }}"
   tags: osds
   become: True
   gather_facts: false
@@ -434,16 +434,20 @@
         name: ceph-facts
         tasks_from: container_binary.yml
 
+    - name: set osd flags, disable autoscaler and balancer
+      run_once: true
+      delegate_to: "{{ groups[mon_group_name][0] }}"
+      block:
         - name: get pool list
           command: "{{ ceph_cmd }} --cluster {{ cluster }} osd pool ls detail -f json"
           register: pool_list
-      run_once: true
           changed_when: false
           check_mode: false
 
         - name: get balancer module status
           command: "{{ ceph_cmd }} --cluster {{ cluster }} balancer status -f json"
           register: balancer_status_update
+          run_once: true
           changed_when: false
           check_mode: false
 
@@ -562,7 +566,7 @@
 
 
 - name: complete osd upgrade
-  hosts: "{{ mon_group_name|default('mons') }}[0]"
+  hosts: "{{ osd_group_name | default('osds') }}"
   tags: osds
   become: True
   gather_facts: false
@@ -573,6 +577,10 @@
         name: ceph-facts
         tasks_from: container_binary.yml
 
+    - name: unset osd flags, re-enable pg autoscaler and balancer
+      run_once: true
+      delegate_to: "{{ groups[mon_group_name][0] }}"
+      block:
         - name: re-enable pg autoscale on pools
           ceph_pool:
             name: "{{ item.name }}"