nfs: AUTH support for exported sub-directories

Problem: NFS allows exporting subdirectories but there is not support for
providing AUTH on per directory basis.

Fix: Modified nfs.export-dir to include AUTH parameters
 e.g. nfs.export-dir "/dir1(10.1.1.2),/dir2(10.1.1.0/24|host1)

During mount operation NFS will check if the IP from where the connection is made
is configured in the AUTH parameter, else the mount operation will fail with
EACCES error.

Updated admin-guide and volume set help message.

Change-Id: I5c6d22edb168b4f46376d1cd6878cd065fc081cc
BUG: 968227
Signed-off-by: Rajesh Joseph <rjoseph@redhat.com>
Reviewed-on: http://review.gluster.org/5124
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>