test/ocf: wait for spdk app to finish at the end

A termination signal was being sent, but we didn't
wait for the spdk app to actually exit. This was
actually causing an intermittent failure on our CI,
as the application could exit during our setup.sh
cleanup call, giving the following error:

```
Removing:    /dev/shm/iscsi_trace.pid284533
rm: cannot remove '/dev/shm/iscsi_trace.pid284533': No
such file or directory
```

Change-Id: Ic6ff0130b6264fa506c367d589853e5f3132c1d2
Signed-off-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/450032
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Vitaliy Mysak <vitaliy.mysak@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>

test/ocf/management/multicore.sh

@@ -16,7 +16,7 @@ function start_spdk()
 }
 function stop_spdk()
 {
-	kill -2 $spdk_pid
+	killprocess $spdk_pid
 	trap - SIGINT SIGTERM EXIT
 }