OpenLDAP: Simplify install & config instructions

What does this Merge Request do?

While onboarding, I noticed the "detect the location" TODO that seemed doable for me and stumbled over the fact that gitlab/config/gitlab.yml.example has several ldap: section, only one of which these instructions seem to pertain to.

Hence, how about these updates to the run-slapd script & the README? I tested the latter on macOS and Ubuntu 18.04.4 LTS and get slapd starting. So, the $(uname) == construct seems to work. I combined 2 answers to https://stackoverflow.com/q/394230 for this.

Additionally worth discussing, IMHO

Since some content is duplicated in /doc/howto/ldap.md, shall we merge the rest into gitlab-openldap/README and point howto/README.md#special-topics to it (cc @eread & @dlouzan, because you previously worked on that file)? Or vice versa?

Merge Request checklist

• [-] Tests added for new functionality. If not, please raise Issue to follow-up.
• [-] This change is backward compatible. If not, please include steps to communicate to our users.
• [-] gdk doctor test added, if needed.

Presumably not relevant, because this affects only setup instructions.

• Documentation added/updated, if needed.