Auto-assignment of iodevices should only consider heap memory for determination

When iodevice auto-assignment occurs on first boot, the memory and available cores are considered. For memory, we allow a maximum of one iodevice per GiB of quota. Based on out of memory failures observed in production, I would suggest this be updated to be one iodevice per GiB of reserved heap (at the most), to hopefully help reduce these low memory failures.

This was discussed at a team meeting, and we decided to use this alternate calculation for memory quotas < 8GiB; for memory quotas >= 8GiB, we will continue to allocate as we do today.