Handling missing openssh or VNC service files

- bnc#894419
- If a service file is missing, firewall proposal doesn't configure that service
- And it also doesn't show the configuration in UI (proposal, UI for changing the settings)
- There is a small test for this client created by @ancorgs and the test passes
- I've also checked that manually in installer
- I've failed to move the stuff to separate lib/network file as too many changes were required and I'm running out of time
It's like ... two to four services? Anyway, good idea.