[Beowulf] mandatory use of command qsub

[Mark Hahn]

> How to prevent users run tasks without qsub command?

set rlimits on your login nodes.  we currently have the following in our 
login nodes' /etc/security/limits.conf:

*	hard	core		0
*       hard    cpu             60      # cpu-minutes
*       hard    nproc           200
*       hard    nofile          512
*       hard    as              2000000 # half phys mem

note that linix doesn't enforce rss, so limiting as (vsz) is usually a
decent substitute.