Automatic merge from submit-queue (batch tested with PRs 38294, 37009, 36778, 38130, 37835)

Only configure basic auth on gci if KUBE_USER and KUBE_PASSWORD are specified.

This should not change the existing flow when KUBE_USER/KUBE_PASSWORD are specified.
It makes not specifying those a valid option that means "don't turn on basic auth".
I only did it for cluster/gce/gci for now, but others should be somewhat similar.