hack/single-node: Update bootkube-up to add kubeconfig once (#94)
hack/single-node: Update bootkube-up to add kubeconfig once (#94)

This is to match multi-node. This was causing TLS errors because it
caused the user-data containing the kubeconfig to be updated on every
bootkube-up run, when it should only happen on the first run.
hongchaodeng/bootkube,kubernetes-incubator/bootkube,kubernetes-incubator/bootkube,dghubble/bootkube,diegs/bootkube,ericchiang/bootkube,diegs/bootkube,diegs/bootkube,dghubble/bootkube,kubernetes-incubator/bootkube,ericchiang/bootkube,hongchaodeng/bootkube,dghubble/bootkube,ericchiang/bootkube,hongchaodeng/bootkube