engine: Clean up orphan gluster task references

Gluster tasks sync job periodically updates status
of gluster tasks in engine. However, if for some reason
(for instance, if engine was offline for a period of time)
gluster does not return a task's information, engine should
mark the job ended with task status UNKNOWN

Change-Id: I61f921109fb710dfb3f7255b54595c882c6a076c
Bug-Url: https://bugzilla.redhat.com/1022996
Signed-off-by: Sahina Bose <sabose@redhat.com>