(PUP-3756) Only chown log file when running as root

Previously, if you specified a log file destination on non-windows
platforms, puppet would attempt to chown the file to the `Puppet[:user]`
and `Puppet[:group]`. However, this will only work if puppet is running
as root.

This commit change the log file destination to only chown when running
as root. It continues to skip the operation on Windows.