Add ceph_mons to ceph example

The ceph documentation backported to Pike does not include ceph_mons
in the vars example, because the var does not need to be defined in
master.

In Pike however, it does need to be defined, and ceph_client will not
run unless it is defined by the user. Once
Ida44228c2d2919990bac46936317b6093d3ff115 has merged, this commit
can be reverted.

Change-Id: Ia6218dbc90af3e379af28bff5cc957dd3feb81d0

etc/openstack_deploy/user_variables.yml.prod-ceph.example

@@ -13,6 +13,11 @@ haproxy_keepalived_internal_interface: br-mgmt
 generate_fsid: false
 fsid: 116f14c4-7fe1-40e4-94eb-9240b63de5c1 # Replace with your generated UUID
 
+## Provide the ceph_mons list which is used by the OSA ceph_client role
+ceph_mons: "{{ groups[mon_group_name]
+               | map('extract', hostvars, 'ansible_host')
+               | list }}"
+
 ## ceph-ansible settings
 ## See https://github.com/ceph/ceph-ansible/tree/master/group_vars for
 ## additional configuration options availble.