[Beowulf] SSH without login in nodes

[Mark Hahn]

> Yes, this is the other solution.  Do nothing fancy in script-land.  Just

right on!  we have >1800 users from more institutions than I can count,
and we just tell users to submit everything nontrivial through the 
queueing system.  only rarely do we need to scold users for running 
nontrivial jobs on login nodes, and even more rarely do any of them 
mess with compute nodes directly.  we make no effort to actually 
_prevent_ users from ssh'ing to compute nodes.

IMO, part of this is making it easy to submit jobs.  in our environment,
just prefix your command by "sqsub" and it goes onto a compute node.
(that is, extra job scripts are a mistake, and it's important to propogate
cwd and env transparently to jobs.)  if compilers took more than milliseconds
to run, people could certainly define FC="sqsub f90" if they wanted, and it
would work correctly.