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

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'.

Change-Id: Ie805b9dd52fd82fd277c3da9ee91cc8b6dea8212
BUG: 1006813
Signed-off-by: Timothy Asir <tjeyasin@redhat.com>
Reviewed-on: http://review.gluster.org/4950
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Kaushal M <kaushal@redhat.com>