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

Change-Id: I674904d380b5f8300e9ca9e6af557c3d30d6cff4
BUG: 989846
Signed-off-by: shishir gowda <sgowda@redhat.com>
Reviewed-on: http://review.gluster.org/5399
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>