Changed to use network.service alias link instead of NETWORKMANAGER=yes/no

- Changed to use network.service alias installed by the NetworkManager
    service file while "systemctl enable".
    Obsoletes the use of the NETWORKMANAGER=yes/no sysconfig variable in
    /etc/sysconfig/network/config (bnc#764055,bnc#764336,bnc#798348).
- Added Service::GetUnitId and Service::GetServiceId utilities