Automatic merge from submit-queue

log-dump.sh: Add a LOG_DUMP_USE_KUBECTL mode, refactor

**What this PR does / why we need it**: This refactors log-dump.sh so that it can optionally just use `"kubectl get node"` output, for dumping logs off a cluster that was brought up using something other than kube-up.sh (e.g. kops or kubeadm).

Note that for obvious reasons, if the cluster is fried, it'll probably fail to hit the apiserver and all bets are off, but for the 95% case where you're debugging tests, this is good. To cover the remaining 5%, you could actually squirrel away the relevant external IPs after the e2e.go:Up stage (because we better be able to hit the apiserver then) and fetch them from cache later.

**Special notes for your reviewer**: Sorry for so many changes. :)