Implement a haproxy box in the Vagrantfile for exercising rolling update scenarios

[tima]

In order to support rolling update examples like #158, the Vagrantfile needs an haproxy machine. It used to at one point:

cluster.vm.define "haproxy" do |config|
  config.vm.box = "centos/7"
  config.ssh.insert_key = false
  config.vm.provider :virtualbox do |vb, override|
    vb.customize ["modifyvm", :id, "--memory", "256"]
    vb.customize ["modifyvm", :id, "--cpus", "1"]
  end
  config.vm.hostname = "haproxy"
  config.vm.network :private_network, ip: "10.42.0.100"
end

In addition this node should be added to the static inventory file in it's own group.

Dependent on #268 getting merged.