aws-cluster: Update to latest CoreOs ami
aws-cluster: Update to latest CoreOs ami

Instances were auto-rebooting in an attempt to upgrade.  We need a
better story around this, but for now, we'll just update to the latest
version.

stevenlin598/quilt,kklin/quilt,secant/quilt,secant/quilt,secant/quilt,stevenlin598/quilt,quilt/quilt,LuiseV/quilt,kayousterhout/quilt,ejj/quilt-dev,kklin/kelda,LuiseV/quilt,mmussomele/quilt,ejj/quilt-dev,stevenlin598/quilt,PenguinToast/quilt,tejasmanohar/quilt,kayousterhout/quilt,tejasmanohar/quilt,stevenlin598/quilt,akshayknarayan/quilt,ejj/quilt-dev,mmussomele/quilt,ZhongtianWang/quilt,ZhongtianWang/quilt,tejasmanohar/quilt,ZhongtianWang/quilt,ejj/quilt-dev,LuiseV/quilt,akshayknarayan/quilt,stevenlin598/quilt,mmussomele/quilt,kklin/kelda,kklin/quilt,kayousterhout/quilt,PenguinToast/quilt,quilt/quilt,quilt/quilt,quilt/quilt,tejasmanohar/quilt,mmussomele/quilt,kelda/kelda,NetSys/quilt,kklin/quilt,NetSys/quilt,kklin/kelda,kelda/kelda,akshayknarayan/quilt,LuiseV/quilt,mmussomele/quilt,stevenlin598/quilt,kklin/kelda,NetSys/quilt,PenguinToast/quilt,secant/quilt,kklin/quilt,kelda/kelda,secant/quilt,NetSys/quilt,ZhongtianWang/quilt,mmussomele/quilt,PenguinToast/quilt,kayousterhout/quilt,LuiseV/quilt,akshayknarayan/quilt,kelda/kelda,NetSys/quilt,PenguinToast/quilt