4380: Reorganize arvnodeman.computenode.

This makes the hierarchy a little richer:

* arvnodeman.computenode.driver has all the cloud driver wrappers.
* arvnodeman.computenode.dispatch will be just like that, except it
  will consider local dispatch concerns.  For example, I'm going to
  add a SLURM submodule here to take care of draining.
* arvnodeman.computenode still has utility functions and
  ShutdownTimer.
Merge branch '4380-node-manager-computenode-reorg-wip'

Refs #4380.  Closes #4496.