SystemExit exception is usually raised by signal handlers
and then caught in main thread to handle signal. Signal
handler executes in main threads but if we use eventlet
we have ony one thread and signal handler can interrupt
any greenlet. Therefore we need to re-raise this exception
to catch it in main greenlet at last.
Also it is necessary to keep executor working and continue
processing scheduled tasks because signal handling logic may
wait for some job results
Change-Id: Ia920922d8444c8216b442577c37710538ed8e919