gluster: consider rebalance failed case in stop rebalance

  Considering the rebalance failed case in stop rebalance
BLL and update the task status correctly. Mark the task status
same as the status returned from stop rebalance vdsm. If status
is empty, mark the status as aborted.

  Refactored the GlusterTaskUtils.isTaskSuccess() to accept
JobExecutionStatus instead of step.

Change-Id: I147c43c6925b8a52cf8a7667a5d7f101969f5368
Bug-Url: https://bugzilla.redhat.com/1019598
Signed-off-by: Ramesh Nachimuthu <rnachimu@redhat.com>