nfs: exit when all volumes are disabled

Instead of triggering 4-5 error logs, when nfs is
disabled for all volumes, exit the process.

backport from 3.5

Change-Id: I191294232f5848a4a01609b477289d628cf89ba5
BUG: 1095330
Signed-off-by: Kaleb S. KEITHLEY <kkeithle@redhat.com>
Reviewed-on: http://review.gluster.org/7699
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Pranith Kumar Karampuri <pkarampu@redhat.com>
Tested-by: Pranith Kumar Karampuri <pkarampu@redhat.com>