[CI] Add OpenLDAP integration tests

This PR sets up an additional set of test environments that installs OpenLDAP and runs special integration tests that connects against that OpenLDAP server.

This was based off of similar work in github/github-ldap: https://github.com/github/github-ldap/pull/48.

Will flesh out the integration tests today, still more to come.

cc @jch @schaary 

In travis, yes, I think we should always run with integration tests. We can keep other tests running fast so contributors can get a quick sanity check before they make a PR, but then let Travis do the heavy lifting with different rubies and ldap servers.