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
how performance hard it is? maybe it make sense to cache it with
```
  @all_known_services ||= SuSEFirewallServices.all_services.keys

  (services - @all_known_services).empty?
```
as bonus I use intersection of arrays, in base it removes from services all known services and if there is none, then all services are known :)