Use a cheaper way to ping the Kubernetes API in the health check endpoint

After #149, the health check server (`/healthz`) will be listing all namespaces in the cluster as a way to ping the Kubernetes API to determine whether it's available. This method of checking availability is more heavyweight than it has to be, since the API server should also have a `healthz` endpoint. Some code similar to the following should be used to check that endpoint:

``` go
// kubeClient is a *(k8s.io/kubernetes/pkg/client/unversioned).Client
res := kubeClient.Get().AbsPath("/healthz").Do()
if res.Error() != nil{
// not possible to reach the server
}
```

This idea was first proposed by @aledbf in https://github.com/deis/builder/pull/149/files#r52692363