Add HAProxy as internal loadbalancer (#4480)

docs/ha-mode.md

inventory/sample/group_vars/all/all.yml

roles/download/defaults/main.yml

roles/kubernetes/node/defaults/main.yml

roles/kubernetes/node/tasks/haproxy.yml

roles/kubernetes/node/tasks/main.yml

roles/kubernetes/node/tasks/nginx-proxy.yml

roles/kubernetes/node/templates/haproxy.cfg.j2

...s/kubernetes/node/templates/manifests/haproxy.manifest.j2

...bernetes/node/templates/manifests/nginx-proxy.manifest.j2

roles/kubernetes/node/templates/nginx.conf.j2

roles/kubespray-defaults/defaults/main.yaml

roles/network_plugin/contiv/tasks/main.yml

tests/files/gce_ubuntu-flannel-ha.yml