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 
Users should be able to specify the update rate on the command line, especially given that this doesn't check pod health or readiness. If this just blazes through the pods, it might as well not be a rolling update.