In fstab you need to put:
manager1,manager2,manager3:/ /var/data ceph name=admin,secret=[cephkey],noatime,_netdev 0 0
[cephkey] is generated by sudo ceph-authtool -p /etc/ceph/ceph.client.admin.keyring.
In Debian Buster you must install ceph-common.
Sorry for my bad english… I hope I have helped.
@funkypenguin, you forgot add secret option at fstab in to the recipe.