qa: fix teardown in cephtool's test_mon_mds

This was missing from 17b5fc9a but we didn't notice
because the test wasn't being run by the gitbuilders.

Signed-off-by: John Spray <john.spray@redhat.com>
Merge pull request #3065 from ceph/wip-cephtool-exec

qa: fix cephtool tests not getting run

Reviewed-by: Loic Dachary <ldachary@redhat.com>