Patch [1] for switched to use eventlet Event for loopingcall events.
It may now happen that stop event is sent when other event was
already sent and loopingcall is already not running.
That cause AssertionError in eventlet.event module.
To avoid that, we should check if if loopingcall is
running before sending _abort.set().
[1] https://review.openstack.org/#/c/611807/
Closes-Bug #1800879
Change-Id: I28ad3bdb51a20350c90dee4420058c30946897e5