rolling_update: remove default filter on mds group

There's no need to use the default filter on active/standby groups
because if the group doesn't exist then the play is just skipped.

Currently this generates warnings like:

[WARNING]: Could not match supplied host pattern, ignoring: |
[WARNING]: Could not match supplied host pattern, ignoring: default([])

Signed-off-by: Dimitri Savineau <dsavinea@redhat.com>

infrastructure-playbooks/rolling_update.yml

@@ -626,7 +626,7 @@
 - name: upgrade active mds
   vars:
     upgrade_ceph_packages: True
-  hosts: active_mdss | default([])
+  hosts: active_mdss
   become: true
   tasks:
     - import_role:
@@ -667,7 +667,7 @@
 - name: upgrade standbys ceph mdss cluster
   vars:
     upgrade_ceph_packages: True
-  hosts: standby_mdss | default([])
+  hosts: standby_mdss
   become: True
 
   tasks: