[jjo] add kube-router support (#3339)

* [jjo] add kube-router support

Fixes cloudnativelabs/kube-router#147.

* add kube-router as another network_plugin choice
* support most used kube-router flags via
  `kube_router_foo` vars as other plugins
* implement replacing kube-proxy (--run-service-proxy=true) via
  `kube_proxy_mode: none`, verified in a _non kubeadm_enabled_
  install, should also work for recent kubeadm releases via
  `skipKubeProxyInstall: true` config

* [jjo] address PR#3339 review from @woopstar

* add busybox image used by kube-router to downloads

* fix busybox download groups key

* rework kubeadm_enabled + kube_router_run_service_proxy

- verify it working ok w/the kubeadm_enabled and
  kube_router_run_service_proxy true or false

- introduce `kube_proxy_remove` fact, to decouple logic
  from kube_proxy_mode (which affects kubeadm configmap
  settings, thus no-good to ab-use it to 'none')

* improve kube-router.md re: kubeadm_enabled and kube_router_run_service_proxy

* address @woopstar latest review

* add inventory/sample/group_vars/k8s-cluster/k8s-net-kube-router.yml

* fix kube_router_run_service_proxy conditional for kube-proxy removal

* fix kube_proxy_remove fact (w/ |bool), add some needed kube-proxy tags on my and existing changes

* update kube-router tolerations for 1.12 compatibility

* add PriorityClass to kube-router DaemonSet

roles/kubernetes/node/templates/kubelet.standard.env.j2

@@ -124,7 +124,7 @@ KUBELET_HOSTNAME="--hostname-override={{ kube_override_hostname }}"
 
 KUBELET_ARGS="{{ kubelet_args_base }} {{ kubelet_args_dns }} {{ kubelet_args_kubeconfig }} {{ kube_reserved }} --node-labels={{ all_node_labels | join(',') }} {% if kube_feature_gates %} --feature-gates={{ kube_feature_gates|join(',') }} {% endif %} {% if kubelet_custom_flags is string %} {{kubelet_custom_flags}} {% else %}{% for flag in kubelet_custom_flags %} {{flag}} {% endfor %}{% endif %}{% if inventory_hostname in groups['kube-node'] %}{% if kubelet_node_custom_flags is string %} {{kubelet_node_custom_flags}} {% else %}{% for flag in kubelet_node_custom_flags %} {{flag}} {% endfor %}{% endif %}{% endif %}"
 
-{% if kube_network_plugin is defined and kube_network_plugin in ["calico", "canal", "flannel", "weave", "contiv", "cilium"] %}
+{% if kube_network_plugin is defined and kube_network_plugin in ["calico", "canal", "flannel", "weave", "contiv", "cilium", "kube-router"] %}
 KUBELET_NETWORK_PLUGIN="--network-plugin=cni --cni-conf-dir=/etc/cni/net.d --cni-bin-dir=/opt/cni/bin"
 {% elif kube_network_plugin is defined and kube_network_plugin == "weave" %}
 DOCKER_SOCKET="--docker-endpoint=unix:/var/run/weave/weave.sock"