Implicit flushing is not  always enabled, depending on OS, resulting in Script Server result issues

# Describe the bug

On certain PHP releases and on certain OS versions, ob_implicit_flush() is not enabled by default, and with cmd.php, this can result in broken graphs.

## To Reproduce

Ubuntu 18.04 experiences this issue with cmd.php only.  Spine data collection appears fine.

## Expected behavior

Cacti should force implicit flush so these problems can not happen in the future.