Added option for specifying synced_folder vm option

Added option for specifying synced_folder vm option when using custom prefab for vagrant virtualbox.

To accomplish mounting the project's root at /guest/mount/path:

Add to .nodelist.yml:

 ---
 default_set: 'centos-64-x64-cluster'
 sets:
   'centos-64-x64-cluster':
     default_node: 'node1.mydomain.com'
     nodes:
       'node1.mydomain.com':
          prefab: 'centos-64-x64'
          custom_prefabs_path: '.'
...

Example in a project's .prefab.yml

---
 'centos-64-x64':
   provider_specifics:
     vagrant_virtualbox:
       synced_folder: '"../../../", "/guest/mount/path"'