Fix NFS mount

[Gasol]

I don't know where does NFS constant come from, So I assume it would be nfs as literal string.

Here were the error message and storage settings I met:

nebula=> SELECT * FROM storages;
 id |                                    settings
----+---------------------------------------------------------------------------------
  1 | {"path": "127.0.0.1:/mnt/production", "title": "production", "protocol": "nfs"}
  2 | {"path": "127.0.0.1:/mnt/playout", "title": "playout", "protocol": "nfs"}
(2 rows)

2020-10-06 09:43:03 ERROR       nebula          Exception!

    Traceback (most recent call last):
      File "/opt/nebula/nx/agents.py", line 40, in run
        self.main()
      File "/opt/nebula/nx/storage_monitor.py", line 57, in main
        self.mount(storage)
      File "/opt/nebula/nx/storage_monitor.py", line 96, in mount
        elif storage["protocol"] == NFS:
    NameError: name 'NFS' is not defined

[martastain]

Of course. It actually means no one used NFS since version 5 was released. Thanks.