glusterd: Don't reset rebalance status on add-brick

 Backport of 67c28b19355c47e96d1420405cc38753a3e5f9be from master

The rebalance status was being reset to 'Not started' when add-brick was
performed. This would lead to odd cases where a 'rebalance status' on a
volume would show status as 'not started' but would also include the
rebalance statistics. This also affected the showing of asynchronus task
status in 'volume status' command.

By not resetting the status prevent the above issues from happening.
Since we use the running/not-running of the rebalance process as the
check when performing other operations we can safely leave the rebalance
stats collected on an add-brick.

BUG: 1006247
Change-Id: Idade88d9e5a6f27659490b3e6d85495d426ef0a3
Signed-off-by: Kaushal M <kaushal@redhat.com>
Reviewed-on: http://review.gluster.org/5971
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>