Bug 1205869

Imbalance in tiers in Swift was previously addressed by weights. However, no matter what the numerical ratio of weight is set, at a certain point there are not enough devices and replicas remaining in lower weight tiers to balance out the crowding at the higher weight tier. At this moment, the tier becomes underutilized, while an administrator may need to force more than one replica into tier to achieve utilization. The ratio of more-than-1 partition is the overload parameter. This update permits administrators to store more than one replica in a tier in case of severely unbalanced clusters. As a result, it is now possible so sacrifice data durability in order to achieve better utilization, which in some cases is required for availability. For example, a cluster will fail to store new data if low-weight tiers overflow and quorum fails.