find the most current child of a Rollout typically we should only find one, but perhaps a previous reconciliation failure could cause us to find multiple if we do see older ones, recycle them.

get the name of the child whose parent is "rolloutObject" and whose upgrade state is "upgradeState" if none is found, create a new one if one is found, create a new one if "useExistingChild=false", else use existing one.

return whether we're done, and error if any.