Make listen URIs configurable

[larsks]

Previously, slapd in this container would always listen on only ldaps:/// and ldap:///. As indicated in HACKING.md, this makes it difficult to make configuration changes to cn=config. With this change, the user may set OPENLDAP_LISTEN_URIS to change the list of endspoints to which slapd will listen, thus permitting the use of ldapi:/// if so desired. E.g. docker run -e OPENLDAP_LISTEN_URIS="ldapi/// ldaps:///" ....

This also restricts slapd to ldapi:/// when performing the initial configuration steps.

[mrogers950]

@larsks I'm not sure if we care too much to take changes to the default run script unless it somehow impacts OpenShift testing. We use images built from the files in this this repo to bootstrap the OpenShift CI LDAP tests where we mount in our own startup scripts and LDAP data.. Also, we are not actively publishing updated images right now.

[larsks]

Eh, that's okay. It's a minor change with no operational impact and it made the image useful to me. Feel free to close this if you don't want the change.

[enj]

@mrogers950 has confirmed to me that this will not break his testing. @mfojtik please merge.

/lgtm

[openshift-ci-robot]

@enj: changing LGTM is restricted to collaborators

In response to [this](https://github.com/openshift/openldap/pull/22#issuecomment-521680479): >@mrogers950 has confirmed to me that this will not break his testing. @mfojtik please merge. > >/lgtm Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes/test-infra](https://github.com/kubernetes/test-infra/issues/new?title=Prow%20issue:) repository.

[mfojtik]

Merging per request.