Change control plane to use RolloutAfter instead of UpgradeAfter

[richardcase]

Currently the KThressControlPlane has a field called UpgradeAfter thats used to indicate that an update should be performed after a time.

This is similar to the RolloutAfter filed that KCP uses.

It would be good to bring KThressControlPlane inline with KCP, especially considering the request to add support for clusterctl rollout (#80).

As we haven't released v1beta1 yet it would be good to get this into this API version bump instead of the waiting until the next bump.