tests: fix test script to turn on write-behind

Change-Id: I8a3ddc8183355236ff7725229441e27bbf8188e3
BUG: 927146
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/5406
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>