mgmt/glusterd: Enable write-behind in nfs

We observed that the number of write requests thus inodelks
are increasing very rapidly to thousands without write-behind
in the graph.

Change-Id: Id71c9c2b0a4c9601a4644a58a933221c62dab0c0
BUG: 928341
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/4734
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>