Pre-requisite:
- keyring /etc/ceph/ceph.client.nagios.keyring with owner root:nagios, permissions 0640, with the following caps:

  caps: [mon] allow r
  caps: [osd] allow r
  caps: [rbd] allow r

  (command: ceph auth get-or-create client.nagios mon 'allow r' osd 'allow r' rbd 'allow r')

- /etc/ceph/ceph.conf readable by nagios (ie 0644)
