nfs: gluster volume set help shows null as default value

Bug(967445): The default value for all nfs options is displayed as "(null)"

Fix: Changed nfs options to show default value.

Change-Id: I3b1f27439c19a6655f7dcc7891df40706db9e474
BUG: 967445
Signed-off-by: Rajesh Joseph <rjoseph@redhat.com>
Reviewed-on: http://review.gluster.org/5098
Reviewed-by: Santosh Pradhan <spradhan@redhat.com>
Reviewed-by: Kaleb KEITHLEY <kkeithle@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>