glusterd: Added the validation function for subvols-per-directory

Change-Id: Ie2259023b9001311a2032792639c3093054f6750
BUG: 896431
Signed-off-by: Avra Sengupta <asengupt@redhat.com>
Reviewed-on: http://review.gluster.org/4552
Reviewed-by: Jeff Darcy <jdarcy@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>