mon: fix mds pool creation

It is not enough to check for the mds to exists, it actually always does
because we declare the variable. So we need to make sure that there is a
mds host.

Signed-off-by: Sébastien Han <seb@redhat.com>

roles/ceph-mon/tasks/main.yml

@@ -14,6 +14,7 @@
   when:
     - not mon_containerized_deployment
     - groups[mds_group_name] is defined
+    - "{{ groups[mds_group_name]|length > 0 }}"
 
 - include: secure_cluster.yml
   when: