Added:

    /**
     * Indicates the directory in which the unpacked install artifacts can be found. For example, for Tomcat if INSTALL_DIR is 
     * {@code /path/to/installdir}, then the tgz file will be downloaded to {@code /path/to/installdir/apache-tomcat-7.0.47.tar.gz}.
     * When that is unpacked, the actual files will (normally) be found in {@code /path/to/installdir/apache-tomcat-7.0.47/}.
     * The EXPANDED_INSTALL_DIR attribute will be set to this during installation, allowing the appropriate files (e.g. /bin/startup.sh)
     * to be found. 
     * <p>
     * However, an enterprise user may have a bespoke build where the unpacked dir has a different name. If so, the config key
     * could be set to something like:
     *     <pre>
     *     "${(config['install.dir']}/acme-tomcat-1.0.0"
     *     </pre>
     */

Thinking about this more, I'm not sure how well this will cope with someone setting the EXPANDED_INSTALL_DIR configuration. We might just overwrite it in the driver's install method, logging a warning! I'll need to look at that more.