LXC: more compact network configuration

LXC: more compact network configuration

Deduplicated network setup code.
Using 'ip route replace' to replace or add the route.
This strategy can be rolled back safely:
1) add new ip (no harm done, old ip still exists)
2) replace route
  on error:
    Delete the new ip, old one is still in place.
    If deleting the new ip fails, it was already modified
    from within the container, so we only warn about it.
  on success do (3):
3) delete old ip
  New IP + gateway are in place, old IP can be deleted.
  If deletion fails, we only warn like in the error case.