Enable NFS synced folder by default

- Disable the default synced folder implementation
- Enable NFS synced folder by default to omit an inappropriate error message in case of no synced_folder settings in user's Vagrantfile

To tell the truth, I want to disable synced folder by default to omit any error messages.
But it will force users to add `dsabled: false` in `config.vm.synced_folder ".", "/vagrant", type: "nfs"`, if users want to enable it back in their Vagrantfile.
It's not good and not compatible with existing Vagrantfiles.

So I will pull request to fix this behavior of Vagrant.
vagrant/plugins/kernel_v2/config/vm.rb
```ruby
      def synced_folder(hostpath, guestpath, options=nil)
        if Vagrant::Util::Platform.windows?
          # On Windows, Ruby just uses normal '/' for path seps, so
          # just replace normal Windows style seps with Unix ones.
          hostpath = hostpath.to_s.gsub("\\", "/")
        end

        options ||= {}
        options[:guestpath] = guestpath.to_s.gsub(/\/$/, '')
        options[:hostpath]  = hostpath
        **options[:disabled]  = options[:disabled] || false**
        options = (@__synced_folders[options[:guestpath]] || {}).
          merge(options.dup)

        # Make sure the type is a symbol
        options[:type] = options[:type].to_sym if options[:type]

        @__synced_folders[options[:guestpath]] = options
      end
```