How to prevent conversion to APFS on High Sierra install
It should work by exiting any High Sierra installer GUI and starting the install from the command line:

/Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/startosinstall --converttoapfs NO


Further install options can be obtained by using the --usage argument:


  Arguments
  --applicationpath, a path to copy of the OS installer application to start the install with.
  --license, prints the user license agreement only.
  --agreetolicense, agree to license the license you printed with --license.
  --rebootdelay, how long to delay the reboot at the end of preparing. This delay is in seconds and has a maximum of 300 (5 minutes).
  --pidtosignal, Specify a PID to which to send SIGUSR1 upon completion of the prepare phase. To bypass "rebootdelay" send SIGUSR1 back to startosinstall.
  --converttoapfs, specify either YES or NO on if you wish to convert to APFS.
  --installpackage, the path of a package to install after the OS installation is complete; this option can be specified multiple times.
  --usage, prints this message.
  --volume, path to the target volume.   




Sources report that the command (in the Beta version) doesn't do anything on the first execution  but on the second.