glusterd: Added validation function for stripe-block-size.

Change-Id: I050d01b01eac46550aa435da7d96a972e0393d35
BUG: 770655
Signed-off-by: Avra Sengupta <asengupt@redhat.com>
Reviewed-on: http://review.gluster.org/4561
Reviewed-by: Jeff Darcy <jdarcy@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>