[backend] Dependence between worker instances and OBS_SETUP_WORKER_PARTITIONS

Currently, you have to set OBS_SETUP_WORKER_PARTITIONS to something else than "none", so workers are used at all. But if you set it to "none", so that the server LV isn't removed, no workers are prepared because of
```
		if [ ! "0$NUM" -gt 0 ]; then
			exit 0
		fi
```
where `$NUM` isn't set.

In my opinion, the worker instances themselves (`OBS_WORKER_INSTANCES`) shouldn't require OBS_SETUP_WORKER_PARTITIONS!="none".

Do not pull the changes as it is only a first draft / idea, which isn't tested. (on my system, I've just commented out the three lines above)