docs: Document CentOS 8 Stream usage

For the CentOS 8 to 8-stream transition, we will have a period where both
CentOS 8 and 8-stream images are available. We differentiate these images via a
tag - the CentOS 8-stream images will have a tag of victoria-centos8s.

Depends-On: https://review.opendev.org/c/openstack/kolla/+/792763
Change-Id: I481235abfabb9d8fe10c542cf5a012cb7a25e7a7

Author: mnasiadka

doc/source/user/quickstart.rst

@@ -375,6 +375,13 @@ There are a few options that are required to deploy Kolla-Ansible:
 
      kolla_base_distro: "centos"
 
+  You can also use CentOS Stream 8 - it's recommended for new installations
+  because CentOS Linux 8 will be supported only until `31st Dec 2021 <https://wiki.centos.org/About/Product>`__.
+
+  .. code-block:: yaml
+
+     openstack_tag_suffix: "-centos8s"
+
   Next "type" of installation needs to be configured.
   Choices are: