Proper merging of configuration settings.

Refactored configuration according to the guidelines at
  http://docs.vagrantup.com/v2/plugins/configuration.html

Previously, in a multi-VM Vagrantfile, most landrush settings that were
globally set would not be inherited to the VMs, which made it necessary to
re-define them for each and every VM.

The changes in this commit make it possible to define configuration settings
once in a multi-VM Vagrantfile and have them inherited to all the machines, but
still allow the settings to be overriden on a per-VM basis:

    Vagrant.configure(VAGRANTFILE_API_VERSION) do |config|
      # Default Landrush settings for all VMs
      config.landrush.enable
      config.landrush.tld = 'custom.dev'
      config.landrush.guest_redirect_dns = true

      config.vm.define 'box1' do |cfg|
        # Landrush settings will be inherited

        # For this VM:
        #   - landrush is still enabled
        #   - the tld is still 'custom.dev'
        #   - guest_redirect_dns is still true

        # ...
      end

      config.vm.define 'box2' do |cfg|
        # Landrush settings will be inherited and can be overwritten
        cfg.landrush.guest_redirect_dns = false

        # For this VM:
        #   - landrush is still enabled
        #   - the tld is still 'custom.dev'
        #   - but guest_redirect_dns is false for this VM only

        # ...
      end
    end