vagrant: Always mount `node_modules` folder after Vagrant NFS mount

The `/opt/edx/edx-platform/node_modules` needs to be always mounted upon a VM boot, after Vagrant has manually mounted the repository code from the host. Since the mount made by Vagrant isn't part of `fstab`, we wait for `/opt/edx/edx-platform` to be mounted before mounting its subdirectory.

Issue reported in https://groups.google.com/d/msg/edx-code/FK4I2YmduqI/xrX8s0pD620J