gluster: fix cleaning up orphan tasks

When there is rebalance task in progress and
'task list' is failed in vdsm for some reason,
engine should not cleanup this task and move to UNKNOWN.

Change-Id: I4f9bb53e5c2be194efeba966355d0245bcc18384
Bug-Url: https://bugzilla.redhat.com/1060994
Signed-off-by: Kanagaraj M <kmayilsa@redhat.com>