Add CI build for OpenLDAP integration

Early work to add tests using OpenLDAP for #46.

- [ ] install OpenLDAP
- [ ] enable memberOf, refint overlays
- [ ] seed with fixture data
- [ ] write tests specifically for OpenLDAP configuration
- [ ] connect to OpenLDAP instead of booting up ApacheDS for existing tests (fixing fixture mismatchs)

cc @jch 

Fine with me. I don't imagine this section to grow significantly more complex. Both of them are easily readable for me.