Fixed handling the network backend in first stage

- There used to be only Mode.installation but as the same behavior is valid also for update (actually in all cases in first stage), changed to Stage.initial
- Removed running_installer? method, which is not needed anymore
- bnc#879594

I will add a changes file entry when this is approved, there is another PR in review now:

- Fixed network backend handling during upgrade - unified with installation (bnc#879594)