Scheduler: make demand cap a share not a cost

[robertdavidsmith]

Problem

There is currently a problem when adjustedFairShare is capped by demand.

The scheduler caps adjustedFairShare to cost at https://git.c3.zone/GR/armada/blob/69232402e78d1a420d56374415436fe15d2ebf0e/internal/scheduler/context/scheduling.go#L174

However, when deciding whether to evict a job, it compares fair share to actualShare = cost / totalCost at

armada/internal/scheduler/preempting_queue_scheduler.go

Line 132 in 6923240

actualShare := sch.schedulingContext.FairnessCostProvider.UnweightedCostFromQueue(qctx) / totalCost

If totalCost < 1 (which it often is in practice), dividing by totalCost inflates actualShare so that it thinks actualShare is above fair share even for fairly undemanding users. So it evicts many jobs well below fair share.

Solution

Divide cappedShare by totalCost so the adjustedFairShare demand cap is consistent with the eviction logic.