The fix is to restore the <= 5.1 behavior of UBT where the MaxParallelActions argument specifies the maximum number of actions to execute in parallel, not a cap on UBTs heuristic-based selection.
This change also modifies BenchmarkBuild so it will default to the available processor count if no -cores argument is specified. This ensures the behavior of repeated runs will be deterministic as UBT may select different values if the system free memory changes
#jira UE-192236
#rb swarm
#review-26877381 @Joe.Kirchoff
#tests Ran UBT / BenchmarkBuild with arguments that forces the number of actions higher than the locally available core count
[CL 26888230 by Andrew Grant in 5.3 branch]
4cbc7591df