Fix Cluster Scaling When Disk Metric Prioritized
Fix Cluster Scaling When Disk Metric Prioritized

This commit fixes a bug in the `LeastAllocatedNode` method that
results in an error when Replicator attempts to initiate a cluster
scale-in operation if the prioritized scaling metric is set to
`ScalingMetricDisk`.

Closes #240

elsevier-core-engineering/replicator,elsevier-core-engineering/replicator