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

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.

Change-Id: I4c69d9c789d081c6de7e7a81dd0d4eba2e83ec17
BUG: 1006247
Signed-off-by: Kaushal M <kaushal@redhat.com>
Reviewed-on: http://review.gluster.org/5895
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>