Vagrant在CentOS6底下要設定nfs同步本機sharefolder，要在Vagrantfile加上
config.vm.synced_folder "../data", "/vagrant_data", type:"nfs"
還要在VirtualBox要裝Extension，然後vagrant要在裝下面這個plugin才可以
vagrant plugin install vagrant-vbguest
參考：
- https://stackoverflow.com/questions/42074246/vagrant-error-unable-to-mount-virtualbox-shared-folders-guest-additions-vboxs
- https://www.vagrantup.com/docs/synced-folders/nfs.html
