Initial API fields for HA control plane

[pmorie]

Problem

Now that leader election has landed in 0.13, we should make the operator deploy an HA configuration by default.

Persona:

Users of the operator

Exit Criteria
A measurable (binary) test that would indicate that the problem has been resolved.

Time Estimate (optional):
Simple

[evankanderson]

We had a discussion in the WG meeting on 3/3 that we don't want to double the resources for Minikube, so me may need some sort of controllable option and/or auto-sizing (i.e. "turn off HA if there is only 1 node in the cluster").

[pmorie]

I'm renaming this issue to be for the initial API changes that allow you to specify an HA installation.

[pmorie]

After discussion with @jcrossley3, i offer the following proposal:

type KnativeServingSpec struct {
  // other fields omitted

  // non-nil indicates HA is requested
  HighAvailability *HighAvailability
}

type HighAvailability struct {
  // Replicas is the number of replicas to run for the HA parts of the control plane
  Replicas int
}

[pmorie]

part of knative/pkg#1007