site.yaml.sample: do not try to gather facts on "non-ceph" hosts

[insatomcat]

The site.yaml.sample makes ceph-ansible try to gather facts on group "all" which may contains hosts that have nothing to do with ceph-ansible, and/or even unreachable hosts which will make the playbook fail. This commit will replace the "all" group by "ansible_play_hosts", containing only ceph-ansible related hosts.