torture: Use "-o Batchmode=yes" to disable ssh password requests

The torture.sh script normally runs unattended, so there is not much
point in the "ssh" command asking for a password.  This commit therefore
adds the "-o Batchmode=yes" argument to each "ssh" command to cause it
to fail rather than ask for a password.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>