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

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 sometihng 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.

hack/verify-flags/exceptions.txt

@@ -29,8 +29,8 @@ cluster/juju/layers/kubernetes/reactive/k8s.py:    cluster_name = 'kubernetes'
 cluster/juju/layers/kubernetes/reactive/k8s.py:    tlslib.client_key(None, client_key, user='ubuntu', group='ubuntu')
 cluster/lib/logging.sh:      local source_file=${BASH_SOURCE[$frame_no]}
 cluster/lib/logging.sh:    local source_file=${BASH_SOURCE[$stack_skip]}
-cluster/log-dump.sh:    for node_name in "${NODE_NAMES[@]}"; do
 cluster/log-dump.sh:    local -r node_name="${1}"
+cluster/log-dump.sh:  for node_name in "${node_names[@]}"; do
 cluster/log-dump.sh:readonly report_dir="${1:-_artifacts}"
 cluster/mesos/docker/km/build.sh:  km_path=$(find-binary km darwin/amd64)
 cluster/photon-controller/templates/salt-master.sh:  api_servers: $MASTER_NAME