Replace some calls to storage resources with a single call to systemconfiguration

[jameshcorbett]

There are some places in this repo where code looks at storage resources to figure out which rabbits map to which computes. But some rabbits may be down and therefore would not report which compute nodes they are connected to. This can lead to errors at various points. As @matthew-richerson pointed out, the code could check the SystemConfiguration resource instead:

The SystemConfiguration resource is filled in by someone (or some tool) with how the system is supposed to look. The Storages resources give status on how things currently look. The Storages for the missing Rabbits don't show any computes because the Rabbits aren't there to say which computes they can see