Support "no NFS mount" experiments.
We have had the mechanism implemented in the client for some time and available at the site-level or, in special cases, at the node level. New NS command: tb-set-nonfs 1 will ensure that no nodes in the experiment attempt to mount shared filesystems from ops (aka, "fs"). In this case, a minimal homdir is created on each node with basic dotfiles and your .ssh keys. There will also be empty /proj, /share, etc. directories created. One additional mechanism that we have now is that we do not export filesystems from ops to those nodes. Previously, it was all client-side and you could mount the shared FSes if you wanted to. By prohibiting the export of these filesystems, the mechanism is more suitable for "security" experiments.