Automatic merge from submit-queue

Update port forward e2e for go 1.6

Only close the stdout/stderr pipes from kubectl port-forward when we're truly done with the command,
instead of as soon as runPortForward exits.

Also try to gracefully stop kubectl port-forward via SIGINT, instead of always sending SIGKILL, as
this will help avoid spdy goroutine leaks in the Kubelet.

Ref #22149 

cc @smarterclayton @kubernetes/rh-cluster-infra