Add a rollingupdate lib and command to kubectl

First take at a rollingupdate lib/command using approach proposed in #1353. Putting it in kubectl to unblock #2575 and #2576. As I understand it, a decision was made to not have rollingupdate be part of kubectl, but the logic is a bit too involved for a bash script in contrib/rctools/, and adding yet another client tool seems like a usability regression. Happy to discuss.

@bgrant0607, @lavalamp 
This description looks totally wrong.