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
Changes and version
Merge pull request #255 from kobliha/wicked_restart

Fixed handling the network backend in first stage
DonePackage: remove invalid UTF-8 characters (bnc#876459)

to avoid crash

- 3.1.73
recode only when really needed
Merge pull request #256 from yast/invalid_utf8_fix

DonePackage: remove invalid UTF-8 characters (bnc#876459)
Improve restarting of services
Add Service.call method
Fix tests
Rework the Service.call method, add tests and fix SystemdService#restart
Add comments about error logging
Add params comment
Ensure Service#call returns the original command result
Add test example for unknown service name in Service.call
Update changes and version to 3.1.74
Merge pull request #242 from vmoravec/fix-restart

Add Service.call method and improve restarting of services