Add simple nfs plug-in

There has been an nfsserver plug-in for some time however no
corresponding client-side data is collected.

This was added to sos-2.2 earlier this year. This commit is a
forward port of those changes:

    commit 71066b4a7d4fd97692b50bf231ad6810df5ec3d0
    Author: root <root@rhel6-vm2.(none)>
    Date:   Wed Jul 10 17:11:52 2013 +0100

        Collect additional nfs-client relevant files

        Additional NFS client-side data collection:

            "/etc/nfsmount.conf",
            "/proc/fs/nfsfs/servers",
            "/proc/fs/nfsfs/volumes"

        Signed-off-by: Bryn M. Reeves <bmr@redhat.com>

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>