gluster: Return status after remove brick/rebalance stop

Initially glusterVolumeRebalanceStop and glusterVolumeRemoveBrickStop
returns true/false.

Since rebalance and remove brick operations are async operations, if
rebalance/remove-brick STOP is triggered after these async tasks completed
then vdsm returns success when executing stop command. But since task is
completed engine has to show that task is completed(Now showing STOPPED)

This patch returns status after successful execution of stop command.
engine will check the summary[status] to see the status of task and
display in UI accordingly.

This patch will not affect the backward compatibility since "status"
is not changed in output dict.
'status': {'code': 0, 'message': 'Done'}

Output:
{'hosts': [{'filesFailed': LONG as STR,
            'filesMoved': LONG as STR,
            'filesScanned': LONG as STR,
            'filesSkipped': LONG as STR,
            'name': STRING,
            'status': STRING,
            'totalSizeMoved': LONG as STR},..],
 'status': {'code': 0, 'message': 'Done'},
 'summary': {'filesFailed': LONG as STR,
             'filesMoved': LONG as STR,
             'filesScanned': LONG as STR,
             'filesSkipped': LONG as STR,
             'status': STRING,
             'totalSizeMoved': LONG as STR}}

Possible exceptions with these verbs:
No new exceptions introduced, XmlErrorException was raised when status parsing fails.

GlusterVolumeRebalanceStopFailedException
    code = 4137
    message = "Volume rebalance stop failed"

GlusterVolumeRemoveBrickStopFailedException
    code = 4150
    message = "Volume remove brick stop failed"

GlusterXmlErrorException
    code = 4106
    message = "XML error"

Change-Id: I5b46296c8ec9544644d2ab8fed5f9c12236b7ed0
Signed-off-by: Aravinda VK <avishwan@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/20482
Reviewed-by: Bala.FA <barumuga@redhat.com>
Reviewed-by: Timothy Asir <tjeyasin@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>