[COOK-2339] Make slapd.conf complete configurable with attributes

The cookbook has only a few options which allow changes of openldap service configuration (`slapd.conf`). So in most cases cookbook/templates changes are needed to adjust openldap to your own needs.

All hard-coded values from the template should be replaced by node attributes (with default values). A generic attribute hash should allow setting of additional (for the cookbook unknown) config options.