ceph-disk: fix false positive for gitbuilder

The output of test/ceph-disk.sh is very verbose which is good for
debugging errors. However it sometime contains strings that match
/error:/i which is picked by gitbuilder as a sign that the test fail,
even when the exit code is zero.

Remove from the output the three strings triggering false positive in
gitbuilder.

Signed-off-by: Loic Dachary <loic@dachary.org>