Add new updateStrategy for "MasterLast"

[derekperkins]

Using RollingUpdate on a StatefulSet almost guarantees that there will be at least 2 PlannedReparentShard commands run, and in the worst case scenario where PlannedReparentShard keeps selecting n-1, you could have N reparents for N replicas. It would be much preferable to apply updates to all the slaves first, and only then update the master, guaranteeing a maximum of one reparent.