doc: Move glusterd.vol to extras/ directory

It's actually not documentation.

Change-Id: I30c84abc9a31edb2f849dd2d6ff6d3129d87392d
Signed-off-by: Lubomir Rintel <lubo.rintel@gooddata.com>
Reviewed-on: http://review.gluster.org/5474
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>