Preface in openstackinstallguide

[K Rain Leander]

This bug tracker is for errors with the documentation, use the following as a template and remove or add fields as you see fit. Convert [ ] into [x] to check boxes:

- [ ] This doc is inaccurate in this way: ______
- [x] This is a doc addition request.
- [ ] I have a fix to the document that I can paste below including example: input and output.

https://docs.openstack.org/install-guide/preface.html#operating-systems

appears to be missing links to the operating system specific installation guides for:
openSUSE and SUSE Linux Enterprise Server
Red Hat Enterprise Linux and CentOS
Ubuntu

The descriptions for each include, "This guide..." which presumably would link to the specific guide for each OS, but is not a link.

-----------------------------------
Release: on 2018-04-30 15:23
SHA: 75f02bdf39b42d789e652825817ed9e1bfc234ca
Source: https://git.openstack.org/cgit/openstack/openstack-manuals/tree/doc/install-guide/source/preface.rst
URL: https://docs.openstack.org/install-guide/preface.html

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/569317

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/569317
Committed: https://git.openstack.org/cgit/openstack/openstack-manuals/commit/?id=feda943d1a0f0a621a68adc5caa1e6b9f069da7d
Submitter: Zuul
Branch: master

commit feda943d1a0f0a621a68adc5caa1e6b9f069da7d
Author: chenxing <email address hidden>
Date: Mon Jun 25 06:35:33 2018 -0400

    Update the confusing content

    Change-Id: I3ca54f796d45bb027e056a129a10c3a6b256ce0b
    Closes-Bug: #1769186