Add an option to use NFS for Vagrant rather than SSHFS.

# Changes
 - __Enhancement:__ Adds option to use NFS for mounting Vagrant box, rather than using SSHFS, if `DS_VAGRANT_MOUNT_NFS` environment variable is set.

For review: @mshmsh5000, @sergii-tkachenko 

# Why?
SSHFS has been crashing multiple times a day for me, and I'd rather trade a little bit of VM performance for the "me" performance lost when restarting Vagrant and force-unmounting drives all day. I've been testing this on my own machine for a few days and it seems to be working better for me. If the `DS_VAGRANT_MOUNT_NFS` environment variable is not set, __this will not change anything__ for anyone else.