Automatic merge from submit-queue (batch tested with PRs 56021, 55843, 55088, 56117, 55859). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

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.


/assign @cjcullen