Allow relative paths for $log_file

0c851ce3cd8d44256a3577dd81eaa88614095ac4 changed $log_dir for SCL
deployments. However, $log_file was still absolute. That caused it to
log to a non-existing directory.

This patch makes $log_file accept a relative path. If a relative path is
specified, $log_dir will be prepended. Absolute paths are still used
directly which keeps API compatibility.