Tests: Umount nfs mount before volume stop

Change-Id: Ia1b98ee56bfa562c3f97f1649699f49917ebbfc7
BUG: 1092850
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/7813
Reviewed-by: Krutika Dhananjay <kdhananj@redhat.com>
Reviewed-by: Humble Devassy Chirammal <humble.devassy@gmail.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>