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: I901a6a820eb7b21b413d33e1a0a3420c7f4746a8
BUG: 928341
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/4736
Reviewed-by: Anand Avati <avati@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>