resource: Add IO_DEVICES to pick up GPUs through hwloc

[dongahn]

This PR makes IO hierarchy visible to the scheduler. The sched-side change is in flux-framework/flux-sched#354.

Because this will generally increase the overhead of resource-hwloc module, I can wrap this with an option if desired. Let me know if that's the case.

[coveralls]

Coverage decreased (-0.05%) to 79.433% when pulling 1a5fa1c on dongahn:gpu_from_hwloc into e44b40f on flux-framework:master.

[garlick]

Thanks @dongahn. I think this is prob ok to merge. Let's open an issue to revisit adding module options / flux-hwloc command line options to alter the load behavior.

[dongahn]

@garlick: that works for me.

@trws: if this is ok, could you merge? Then that will allow for testing my sched PR tested on Travis.

[dongahn]

Ok I restarted two CI configurations. Seems they hit transient errors.