glusterd: Added option description, and validation function fields.

In volopt_map_entry table, added option description field, and
option validation function pointer.

Change-Id: I21c6bccd175970592b470ce3ef3f418cb99a5a43
BUG: 903478
Signed-off-by: Avra Sengupta <asengupt@redhat.com>
Reviewed-on: http://review.gluster.org/4535
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>