mds,qa: exit instead of respawn under valgrind

mds,qa: exit instead of respawn under valgrind

valgrind can't handle execve of /proc/self/exe:

    2021-02-27T05:52:37.813 INFO:tasks.ceph.mds.d.smithi073.stderr:==00:01:03:20.556 41218== execve(0x18546740(/proc/self/exe), 0x18546670, 0x133ef310) failed, errno 2
    2021-02-27T05:52:37.813 INFO:tasks.ceph.mds.d.smithi073.stderr:==00:01:03:20.556 41218== EXEC FAILED: I can't recover from execve() failing, so I'm dying.
    2021-02-27T05:52:37.813 INFO:tasks.ceph.mds.d.smithi073.stderr:==00:01:03:20.556 41218== Add more stringent tests in PRE(sys_execve), or work out how to recover.

So configure the MDS to just exit so it can be restarted by QA infra (the
daemon watchdog).

Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>