linter: address syntax errors

This fixes the following error:
```
syntax-check[specific]: The field 'hosts' has an invalid value, which
includes an undefined variable.
```

Signed-off-by: Guillaume Abrioux <gabrioux@ibm.com>

infrastructure-playbooks/cephadm-adopt.yml

@@ -603,7 +603,7 @@
         CEPHADM_IMAGE: '{{ ceph_docker_registry }}/{{ ceph_docker_image }}:{{ ceph_docker_image_tag }}'
 
 - name: adopt ceph mgr daemons
-  hosts: "{{ groups[mgr_group_name] | default(groups[mon_group_name]) }}"
+  hosts: "{{ groups['mgrs'] | default(groups['mons']) | default(omit) }}"
   serial: 1
   become: true
   gather_facts: false

infrastructure-playbooks/shrink-mds.yml

@@ -24,7 +24,7 @@
         tasks_from: container_binary
 
 - name: perform checks, remove mds and print cluster health
-  hosts: "{{ groups[mon_group_name][0] }}"
+  hosts: mons[0]
   become: true
   vars_prompt:
     - name: ireallymeanit
@@ -165,4 +165,4 @@
   post_tasks:
     - name: show ceph health
       command: "{{ container_exec_cmd | default('') }} ceph --cluster {{ cluster }} -s"
-      changed_when: false
+      changed_when: false

infrastructure-playbooks/shrink-mgr.yml

@@ -21,7 +21,7 @@
         msg: gather facts on all Ceph hosts for following reference
 
 - name: confirm if user really meant to remove manager from the ceph cluster
-  hosts: "{{ groups[mon_group_name][0] }}"
+  hosts: mons[0]
   become: true
   vars_prompt:
     - name: ireallymeanit
@@ -130,4 +130,4 @@
   post_tasks:
     - name: show ceph health
       command: "{{ container_exec_cmd | default('') }} ceph --cluster {{ cluster }} -s"
-      changed_when: false
+      changed_when: false

infrastructure-playbooks/shrink-mon.yml

@@ -22,7 +22,7 @@
     - debug: msg="gather facts on all Ceph hosts for following reference"
 
 - name: confirm whether user really meant to remove monitor from the ceph cluster
-  hosts: "{{ groups[mon_group_name][0] }}"
+  hosts: mons[0]
   become: true
   vars_prompt:
     - name: ireallymeanit
@@ -144,4 +144,4 @@
     - name: show ceph mon status
       command: "{{ container_exec_cmd }} ceph --cluster {{ cluster }} mon stat"
       delegate_to: "{{ mon_host }}"
-      changed_when: false
+      changed_when: false

infrastructure-playbooks/shrink-osd.yml

@@ -14,8 +14,8 @@
 - name: gather facts and check the init system
 
   hosts:
-    - "{{ mon_group_name|default('mons') }}"
-    - "{{ osd_group_name|default('osds') }}"
+    - mons
+    - osds
 
   become: True
   tasks:
@@ -23,7 +23,7 @@
 
 - name: confirm whether user really meant to remove osd(s) from the cluster
 
-  hosts: "{{ groups[mon_group_name][0] }}"
+  hosts: mons[0]
 
   become: true
 

infrastructure-playbooks/shrink-rbdmirror.yml

@@ -22,7 +22,7 @@
 
 - name: confirm whether user really meant to remove rbd mirror from the ceph
         cluster
-  hosts: "{{ groups[mon_group_name][0] }}"
+  hosts: mons[0]
   become: true
   vars_prompt:
     - name: ireallymeanit