qa/workunits/cephtool/test.sh: disable dup test for mds respawn cmd

qa/workunits/cephtool/test.sh: disable dup test for mds respawn cmd

we set the CEPH_CLI_TEST_DUP_COMMAND enn var to verify the successful
commands are idempotent. but some of them are just not. among the other
things:
- ceph tell mds.a exit
- ceph tell mds.a respawn
the respawn command restart the mds daemon, its bind port changes and
all run-time status are reset. so strictly speaking, even the from the
point of view of client, this command is not idempotent. further more,
it fails the test, if the client sends the 2nd command too soon. because
the monitor might not able to update the re-spawned mds address before
the client asking for the new fsmap. so the cephfs client will just
use the old address of the specified mds, and hence will send the
request to port no one is listening anymore.

Signed-off-by: Kefu Chai <kchai@redhat.com>