[CI] Wire up retcode OpenLDAP overlay for testing specific server responses

This PR configures the CI integration OpenLDAP server with the retcode overlay (section 12.12 http://www.openldap.org/doc/admin24/overlays.html) in order to write integration tests for specific server responses.

Will write some example integration tests against this.

cc @jch @schaary 

This is so slick.