Better balance our isobuilders' I/O load over all available SSDs

They hit almost exclusively md1. I want their load to be better spread over md1 and md3, at least for their -data and their -libvirt volumes, like I did for isotesters already. I’ll take care of it myself as it seems I’m the one who cares most about this topic.

Parent Task: #5630 (closed)

Original created by @intrigeri on 12606 (Redmine)