storage controller: "home" scheduler stats become stale when changing tenant's preferred AZ

[jcsp]

SchedulerNode holds counts of how many shards are scheduled onto the node as their home location, i.e. the tenant's preferred AZ matches the node's preferred AZ. This count is used to guide scheduling of newly created shards.

These stats become inaccurate when we modify the preferred AZ of a tenant manually. There needs to be some logic in TenantShard.set_preferred_az to adjust them.

This is a mild issue -- it only happens when we do manual intervention, and it only makes scheduling of new shards a bit less optimal (and the chart of home location counts a bit confusing).