Use defaultLinuxLogsLocation for /var/log

Log directory 'logs' in the installation directory is soft-linked to
/var/log/app-name/. Instead of hard coding /var/log use the already
provided variable defaultLinuxLogsLocation.
Add test for defaultLinuxLogsLocation

Add a new test 'debian/log-directory' to check that a custom setting for
defaultLinuxLogsLocation is correctly applied.
Merge pull request #150 from hfs/wip/log-directory

Use defaultLinuxLogsLocation for /var/log