Fix sar plug-in sa_path usage and enable on Debian/Ubuntu

Make sure that all places in the sar plug-in that need to
reference the sar log path do so using the sa_path variable.

This enables the plug-in to work on other distributions by
changing the value of sa_path in a new subclass.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>