cmd/coordinator: discover kube cluster

Find the default Kubernetes cluster and configure a client to talk to it.
Use application default credentials.

Updates golang/go#12546

Change-Id: Ifb1ce57f52f4fbbee3267f8cc3cf02a78146bd5b
Reviewed-on: https://go-review.googlesource.com/14532
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>