(PUP-3339) Confine failed test off of Fedora 19

There seems to be a bug in `usermod` on Fedora 19 which is causing
this acceptance test to fail. The bug does not appear to be present
in Fedora 20. Since Fedora 19 will likely each end of life soon,
just confine the acceptance test off the platform rather than try
to find a work around in puppet.