Adjusting enabled/disabled services in ServicesProposal

- bnc#887688
- sshd service is adjusted by Yast Network but the service status is not exported to AutoYast later and also not handled in AutoYast import - this is a task for ServicesManager
- Added new test - passes
- build also passes
JFYI, API nitpicking: I would prefer a single call like `ServicesProposal.enable_service('sshd', :enabled => enabled)`