driverdisk: Use a single systemd service to start DD UI (#1035663)
    
    This removes the second driver-updates service for network UI startup
    and creates a start_driver_update function that is used by the 3 places
    that driver updates need to be processed.
    
    It also moves the driver-fetch-net to run after kickstart-fetch-net so
    that kickstart driverdisk network sources can be processed in the same
    way as the cmdline network sources.
    
    Related: rhbz#1035663