src/stop.sh: check if ceph-mds daemon(s) exist beforehand

src/stop.sh: check if ceph-mds daemon(s) exist beforehand

qa/tasks/vstart_runner.py runs src/stop.sh and src/vstart.sh if --create
is passed to it. Once in a few times, vstart_runner.py hangs on running
stop.sh. Running "ps -ef | grep ceph" shows that following command is
launched every time vstart_runner.py hangs at stop.sh -

  /usr/bin/python3.9 bin/ceph -c <path-to-ceph-repo>/build/ceph.conf tell mds.* client ls

Every time an instance of vstart_runner.py hangs at execution of
stop.sh, a new "ceph tell mds.* client ls" command is launched. This
doesn't happen when stop.sh is run manually. I suspect some issue lies
between this commmand in stop.sh and Python subprocess module.

Anyways, a simple fix is to not to run this command when ceph-mds daemon(s)
are not present on the system.

Signed-off-by: Rishabh Dave <ridave@redhat.com>