Snap tracks for Kubernetes

[George Kraft]

Hey folks,

We're working on a collection of snaps for Kubernetes that we'd like to
have tracks added to:
- kubectl
- kube-apiserver
- kube-controller-manager
- kube-scheduler
- kubelet
- kube-proxy
- cdk-addons

All of these need tracks for 1.5, 1.6, and 1.7. Can someone create these
for us?

Thanks!
George