drop variables from 'hosts' key

The linter complains about that.
It doesn't work anyway so it doesn't make sense to leave these variables
here.

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

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

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

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

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

@@ -13,8 +13,8 @@
 
 - name: gather facts and check the init system
   hosts:
-    - "{{ mon_group_name|default('mons') }}"
-    - "{{ mon_group_name|default('rbdmirrors') }}"
+    - mons
+    - rbdmirrors
   become: true
   tasks:
     - debug:
@@ -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