Nexenta NFS driver refactoring

Refactor NFS driver, redefined volume_backend_name attribute
inherited from RemoteFsDriver.

Change-Id: I607277053c1233c688dbfaf0c5e150f9511b83af
Remove the use of common.uuidutils.gen_uuid

There's a movement to remove gen_uuid from the
openstack common.  This change removes it's use
from Cinder and just puts the call inline.

Change-Id: I07329bab9553e15e498ddbda3848d50b8f48fd53
Merge "Remove the use of common.uuidutils.gen_uuid"
Merge "Nexenta NFS driver refactoring"