Support tracking NFS mounts

[matsimitsu]

A common use-case for NFS mounts is to store user uploads independently of the front-end server hosting the app, ideally we track the disk usage for these shares too, as it's important to know if there's enough space to store user uploads.

[tombruijn]

@matsimitsu do you have an intercom link?

[vbmade2000]

@matsimitsu Do you mind if I try this issue ?

[vbmade2000]

Following are the ways to get NFS mounts.

1. showmount --exports
   It can be used only if rpcbind is used in NFS. With NFSv4, rpcbind is not used any more so showmount will throw clnt_create: RPC: Program not registered error on NFSv4 server configuration.

2. exportfs -v
   This utility comes with NFS server package (at least in Ubuntu). Client needs to install it separately. There are other native ways to get the NFS mount so for normal case client doesn't have a reason to install it.

3. cat /var/lib/nfs/etab
   Again this file is not available on client system or at least this was the case with my VM. It was seen in server.

4. nfsstat -m
   This utility shows all the mounts for currently mounted NFS.

5. mount | grep nfs
   Regular mount utility that shows all the mounts. We can grep for nfs mount.

I believe that we should mount | grep nfs as the utility is built in. I don't know if there is any issues related with that.

@thijsc @matsimitsu thoughts ?