cluster/dht: Treat migration failures due to space constraints as skipped

Currently rebalance/remove-brick op's display migration failed count even
for files which failed due to space issues (not enough space for file, or
migration leading to cluster imbalance)

These will now be counted as skipped, and rebalance/remove-brick status
will display the additional counter

BUG: 989846
Change-Id: I4efa7ce69dd43680ff47181afed0c561954c5080
Signed-off-by: shishir gowda <sgowda@redhat.com>
Reviewed-on: http://review.gluster.org/5977
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>