Fix enabling services in inst-sys | bnc#870949

In inst-sys the command `systemctl show` does not work. The class `SystemdUnit` is based on information of its output. 
I added a new class which does is responsible for doing the job of `systemctl show`, but only in the installation. It's functionality is pretty limited, but that's what's needed in installation system anyway: 
1. check if a service is enabled
2. enable it if it's not