Specify topology spread constraints on router deployments to encourage the
scheduler to spread router pod replicas across availability zones.

Add a to-do to replace the hard anti-affinity rule with soft anti-affinity
once the descheduler has support for enforcing soft anti-affinity rules.

This commit fixes bug 1900819.

https://bugzilla.redhat.com/show_bug.cgi?id=1900819

* pkg/operator/controller/ingress/deployment.go (desiredRouterDeployment):
Rename needDeploymentHash to configureAffinity as we now always need to
compute the deployment hash but do not always specify an affinity policy.
Specify spec.template.spec.topologySpreadConstraints.  Specify max skew 1,
unsatisfiable-constraint action "ScheduleAnyway", topology key
"topology.kubernetes.io/zone", and a label selector on the deployment hash
label so as to specify a preference for spreading replicas of the same
generation out but not prevent scheduling replicas of different generations
on the same node or scheduling more replicas than there are availability
zones.  Add to-do for changing to soft anti-affinity.
(hashableDeployment): Hash spec.template.spec.topologySpreadConstraints.
(cmpMatchExpressions, zeroOutDeploymentHash): New functions, factored out
of hashableDeployment.
* pkg/operator/controller/ingress/deployment_test.go (checkDeploymentHash):
New function, factored out of TestDesiredRouterDeployment.
(TestDesiredRouterDeployment): Use checkDeploymentHash.  Expect the
deployment hash label always to be set.
(TestDeploymentConfigChanged): Add test cases for
spec.template.spec.topologySpreadConstraints.