Automatic merge from submit-queue

test/e2e/addon_update: Respect KUBE_SSH_USER

This change makes the e2e tests more consistent as the other ones all already respected this variable.

I didn't do the larger change of re-factoring it to use `framework.SSH` because getting the `scp` portion working there has some significant complexity. If I find time, I'd like to go back and do it since this test needs a little ❤

cc @yifan-gu, @kubernetes/sig-testing