Nexenta NFS volume driver folder auto sharing

1. Auto sharing for NFS enclosing folders, this helps to avoid errors if user
forgot to share enclosing directory.

2. Added version history to NexentaNfsDriver doc string.

Change-Id: I9aa2a7b8760c687985aa11d85715188f1c1622f9
Merge "Nexenta NFS volume driver folder auto sharing"