vm_devbox multihost

[e-carlin]

I'd like to solve this. We need it for slurm and for rsconf.

radiasoft/download#620 (comment)

I was starting to work on radiasoft/sirepo#6946 and realized that the work to solve it will probably be undone when we switch to supporting multihost. Better to get it right the first time.

I don't think we need to make it work across host machines. Just vm_devbox's need to be able to communicate with eachother on a single host.

First step is to understand how this works outside of vm_devbox (ex v3 and v4 for rsconf)