nfs: exit when all volumes are disabled

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

Change-Id: Ib286f143c4f74ba22f502aca0e7dcd0907db6563
BUG: 976750
Signed-off-by: Rajesh Amaravathi <rajesh@redhat.com>
Reviewed-on: http://review.gluster.org/5245
Reviewed-by: Santosh Pradhan <spradhan@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>