Allow rescheduling of Pods to different Server Groups if more are available

In public cloud providers, it is common for the Server Groups in use to match the Availability Zones, and for there to be a small number of them. In private data centres they are more likely to be used to represent a larger number of zones, based on racks, rows of racks, or even rooms.

Generally, when rescheduling a Pod, the same Server Group would be used, but if there are more groups than needed then an alternative group could be used if the rescheduling fails in the original group.