cli/glusterd: Support of volume get for a specific volume option

This patch introduces a cli command to display a specific volume option/all
volume options of a specific volume with the following usage:

Usage: volume get <VOLNAME> <key|all>

Change-Id: Ic88edb33c5509d7a37cd5ade6341e45e3cdbf59d
BUG: 983317
Signed-off-by: Atin Mukherjee <amukherj@redhat.com>
Reviewed-on: http://review.gluster.org/8305
Reviewed-by: Kaushal M <kaushal@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>