[CI] Add Vagrant box for running tests against OpenLDAP locally

This allows us to run all of our tests against OpenLDAP locally instead of being restricted to just ApacheDS or having to push up to have TravisCI run tests. This allows faster iteration and better feedback when writing new tests (against OpenLDAP in particular) or when troubleshooting failures.

This is a pretty blatant ripoff of https://github.com/ruby-ldap/ruby-net-ldap/pull/129.

cc @jch