Do not consider iptables' output an error in case of xtables lock

This solves errors starting containers in parallel and ignores this stdout message from iptables:
```
Another app is currently holding the xtables lock; waiting for it to exit...
```

NOTE: the real bug is that somebody considers iptables' stdout to be always a sign of error, however this patch is the minimum necessary to solve the problem at hand. Generally speaking it would be a good idea to *not* consider any stdout from iptables an error (not only while linking containers).

Fixes #6010