## Problem

When we update our scheduler/optimization code to respect AZs properly
(#9916), the choice of AZ
becomes a much higher-stakes decision. We will pretty much always run a
tenant in its preferred AZ, and that AZ is fixed for the lifetime of the
tenant (unless a human intervenes)

Eventually, when we do auto-balancing based on utilization, I anticipate
that part of that will be to automatically change the AZ of tenants if
our original scheduling decisions have caused imbalance, but as an
interim measure, we can at least avoid making this scheduling decision
based purely on which AZ contains the emptiest node.

This is a precursor to #9947

## Summary of changes

- When creating a tenant, instead of scheduling a shard and then reading
its preferred AZ back, make the AZ decision first.
- Instead of choosing AZ based on which node is emptiest, use the median
utilization of nodes in each AZ to pick the AZ to use. This avoids bad
AZ decisions during periods when some node has very low utilization
(such as after replacing a dead node)

I considered also making the selection a weighted pseudo-random choice
based on utilization, but wanted to avoid destabilising tests with that
for now.

## Problem

It is unreliable for the control plane to infer the AZ for computes from
where the tenant is currently attached, because if a tenant happens to
be in a degraded state or a release is ongoing while a compute starts,
then the tenant's attached AZ can be a different one to where it will
run long-term, and the control plane doesn't check back later to restart
the compute.

This can land in parallel with
#9947

## Summary of changes

- Thread through the preferred AZ into the compute hook code via the
reconciler
- Include the preferred AZ in the body of compute hook notifications