[Beowulf] let's standardize liquid cooling

[Bill Broadley]

Sounds expensive, complicated, and challenging.

How about a MUCH simpler proposal: eliminate fans from compute nodes.

Nodes should:
* assume good front to back airflow

Racks would:
* have large fans front AND back that run at relatively low
  rpm, and relatively quiet.
* If front or rear door open the other door would ramp up fans
  (and noise) to compensate.

My measurements of 1U nodes have shown a significant fraction of the
power budget goes to fans!  Fans that fit in 1U (even 2U) nodes seem to
generate more noise, heat, and vibration than airflow.