cluster/dht: In reconfig handle removed decommissioned nodes

If no decommissioned nodes options are set in the options, then
clear the conf->decommissioned_bricks.

Change-Id: I426d2bcc874aab21b2eba0b16a580b9a26672ea2
Signed-off-by: shishir gowda <sgowda@redhat.com>
BUG: 973073
Reviewed-on: http://review.gluster.org/5199
Reviewed-by: Amar Tumballi <amarts@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>