Create a hostlist tree and use it to ssh to the nodes to start/finish tasks

[antoniogi]

Sounds fancier than what it is. Right now, we go node by node, open an SSH connection, and launch a background task or copy files. This does not scale. It's easy to create a tree (not necessarily binary) where each node in the list only opens an SSH connection to its children. This is a relatively big change in the code.