Vagrantfile

...
$num_instances = 3
(1..$num_instances).each do |i|
  config.vm.define "node#{i}" do |node|
    node.vm.box = "centos/7"
    node.vm.hostname = "node#{i}"
    ip = "192.168.8.#{i+100}"
    node.vm.network "private_network", ip: ip
    node.vm.provider "virtualbox" do |vb|
      vb.memory = "3072"
      vb.cpus = 1
      vb.name = "node#{i}"
    end
    node.vm.provision "shell", path: "bootstrap.sh", args: [i, ip]
  end
end
...