Add check to validate that all pods in control plane are in the Ready state

[klingerf]

We already have checks the validate that a) the control plane namespace exists, and b) calling the public-api's SelfCheck endpoint succeeds. Calling the SelfCheck endpoint won't succeed, however, if the prometheus pod hasn't passed it's readiness check yet. So it would be more clear to have have an explicit check to validate that all pods are in the Ready state. We should also be able to retry this check using the --wait flag (#1081).