This is the same fix as bc846b7da6d9af16d9611fef7f00df6557f19def
applied to the other part of the code-base that builds ceph.conf (I'd
missed that 349b9ab3e72f188c50b5f84f3ce5ee2fddcfbf80 had duplicated
this code).

Signed-off-by: Matthew Vernon <mv3@sanger.ac.uk>