vagrant_plugin_install_helper.rb

def install_plugins_if_missing(required_plugins)
  plugins_to_install = required_plugins.select { |plugin| not Vagrant.has_plugin? plugin }
  if not plugins_to_install.empty?
    puts "Installing plugins: #{plugins_to_install.join(' ')}"
    if system "vagrant plugin install #{plugins_to_install.join(' ')}"
      exec "vagrant #{ARGV.join(' ')}"
    else
      abort "Installation of one or more plugins has failed. Aborting."
    end
  end
end