tools/cephfs/first-damage: default conf if CEPH_CONF not set

tools/cephfs/first-damage: default conf if CEPH_CONF not set

If $CEPH_CONF isn't set, you get:

unable to get monitor info from DNS SRV with service name: ceph-mon
2025-05-08T22:40:06.091-0400 7f604286f1c0 -1 failed for service _ceph-mon._tcp
2025-05-08T22:40:06.091-0400 7f604286f1c0 -1 monclient: get_monmap_and_config cannot identify monitors to contact
Traceback (most recent call last):
  File "/root/first-damage.py", line 144, in <module>
    R.connect()
  File "rados.pyx", line 690, in rados.Rados.connect
rados.ObjectNotFound: [errno 2] RADOS object not found (error connecting to the cluster)

Make it default to /etc/ceph/ceph.conf if the env isn't set.

Signed-off-by: Dan van der Ster <dan.vanderster@clyso.com>