Automatically install missing Vagrant plugins upon 'vagrant up'

vagrantfile_ensure_plugins.rb

# Install any missing plugins upon 'vagrant up'
def ensure_plugins(required_plugins)
  if ARGV[0] == 'up'
    missing_plugins = required_plugins.select {|p| not Vagrant.has_plugin? p}
    unless missing_plugins.empty?
      plugins = missing_plugins.join(' ')
      puts "Found missing plugins: #{plugins}, installing..."
      exec "vagrant plugin install #{plugins} && vagrant up"
    end
  end
end