Automatically configure LDAP if OpenLDAP is enabled

What does this Merge Request do and why?

Previously setting openldap.enabled to true would only enable the Procfile, but a number of manual steps were necessary to get LDAP running:

1. make -C gitlab-openldap default
2. Configure gitlab.yml with the main and alternate LDAP groups.

This commit addresses both steps and makes it easy to get LDAP running with the GDK. This would have saved me an hour of debugging.

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.