[Enhancement] Add OVAL support for gconf-tree.xml

- Adds OVAL support for /etc/gconf.xml.mandatory/%gconf-tree.xml
- Update gconftool GDM login banner prose to enforce settings

This supports the GNOME gconf order of precedence which can be found here: https://projects.gnome.org/gconf/. If /etc/gconf.xml.mandatory/%gconf-tree.xml exists, any configuration under /etc/gconf.xml.mandatory will be ignored as well as any configuration under /var.