cli: add aggregate status for rebalance and remove-brick status xml output

 Backport of 91e4b7aa1361087317238b9bf6427ef274737c8c from master

Add aggregate status information in <aggregate> section of gluster volume
'rebalance status' and 'remove-brick status cli xml output.

The aggregate status determined based on the most critical level
and the aggregate status will have 'Complete' only when all
individual status are 'Complete'.

BUG: 1006813
Change-Id: I803d4bfb31dbda53b5665332263576c910562805
Original-author: Timothy Asir <tjeyasin@redhat.com>
Signed-off-by: Kaushal M <kaushal@redhat.com>
Reviewed-on: http://review.gluster.org/5972
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>