Speedup Security.ReadServiceSettings (bnc#890349) 

* Greatly speed up the startup of yast-users and yast-security by avoiding 3 unnecessary calls to SystemdService.all
* Drop use of deprecated Service.EnabledServices
* Drop obsolete runlevel parameter from some methods
* Some basic tests
I changed the interface. The argument is not longer a runlevel (because it not longer makes sense now that we don't use systemv). But you are right, I forgot to update the comment/documentation.