Ensure monitor nodes have the keyring in place

As per documentation:

https://docs.ceph.com/en/latest/cephadm/install/#running-the-bootstrap-command
https://docs.ceph.com/en/latest/cephadm/install/#adding-hosts

Nodes that have '_admin' label applied in spec will have the admin
keyring present in /etc/ceph.

Author: cityofships

roles/cephadm/templates/cluster.yml.j2

@@ -4,6 +4,7 @@ service_type: host
 hostname: {{ hostvars[host]['ansible_hostname'] }}
 labels:
 {% if host in groups['mons'] %}
+- _admin
 - mon
 {% endif %}
 {% if host in groups['mgrs'] %}