Flux: ch-run: non-default binds required

[j-ogas]

Follow-up issue to: #1597

The Flux WLM stores node vadar segment files at /var/tmp/$USER/flux-*, where * appears to be a dynamic URI scheme. Thus, to run with the Flux WLM, users must have their /var/tmp/${USER} directory bind-mounted at run-time.

This is problematic because:

• user may not know they need to bind-mount this directory
• the path is different for each user
• the path needs to exist in the image prior to running; otherwise bind-mounting will fail unless --write is passed
• requires a posteriori knowledge to run without error

[reidpr]

On point 3 at least, /var/tmp is very standard so should almost always be available.