auth/gcp: capture stderr from gcloud config-helper
auth/gcp: capture stderr from gcloud config-helper

Fixes https://github.com/kubernetes/client-go/issues/327

Currently we're not capturing stderr when shelling out to "gcloud config
config-helper --format=json" fails. This patch is capturing the stderr in the
error message returned.

This gcloud command sometimes returns an upgrade reminder (in stderr) so using
`cmd.CombinedOutput()` is not an option here. Maybe we should also add an unit
test in gcp_test.go capturing this situation, too, in case someone decides to
replace this with CombinedOutput someday.

Signed-off-by: Ahmet Alp Balkan <ahmetb@google.com>

Kubernetes-commit: 1334523c73fea3b3d9d0e90f287b3e385c79e6cd

kubernetes/client-go,li-ang/client-go