[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 

@jch are you thinking we should :fire: the `skip` option here and just always run with integration?

Obviously the plan here is to add `activedirectory` as an option at some point, once we work out how to do that.