Add olcChainConfig overlay

[gcoxmoz]

This Pull Request (PR) fixes the following issues

Is a precursor to #431 (makes progress but does not solve the whole issue)

Pull Request (PR) description

4 parts to this PR:

1. [boring] wraps the spec unit test for openldap_overlay in a new 'describe' layer (so we can isolate the existing 'create an overlay' test from new test).
2. [boring] remove an unused parameter from the unit test.
3. [cleanliness] add a 'does .instances work correctly"' test to openldap_overlay. The current tests are "can we make a new overlay" but this adds a class of tests for "given slapcat of an existing deployment, do we detect+parse the overlay correctly?" - richer testing on existing behavior, and a framework for the next commit.
4. [the important commit] Add olcChainConfig and the ability to attach the overlay to the frontend pseudo-database, following a prior-art pattern from the openldap_access provider. ... and tests for adding/detecting the overlay.

#431 wants to add a Chain config, which is a two-part request: adding the Chain overlay, and then adding the database you're chaining TO. However, that second part is more complex changes to openldap_database, and it's all dependent upon having this overlay in place first. So while this isn't the complete solution I think there's enough in here to fold in (because if this gets rejected, the database work is of no value)