Adds NFS exports support into the drupal cookbook.

This allows users to provision NFS shares (exports) on spin-up, essentially providing a route for the proposal in #89 to succeed by way of a simple addition to one's drupal_lamp.json in the drupal-lamp project.

This is built so that it can be used to create NFS exports on the guest of any type for any reason - it makes no assumptions.

To use it to overcome #89, one might:
Define a share in drupal_lamp.json.
Vagrant provision/up.
Mount the share from the host machine.

Exports must be defined in node[:drupal][:server][:nfs_exports].

Example:
````
    "nfs_exports": {
      "/assets": {
        "clients": {
          "192.168.50.1": {
            "writeable": true,
            "sync": true,
            "options": [
              "all_squash",
              "anonuid=33",
              "anongid=33"
            ]
          }
        }
      }
    }
````