[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