* [jjo] add kube-router support
Fixescloudnativelabs/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
