Add docs and release note for CentOS 8

Adds info on supported distros to support matrix. Adds release note
about CentOS 7 drop and dropped images.

Change-Id: I0ada13af2fe57de550c1703d636436190e5d489a
Partially-Implements: blueprint centos-rhel-8

doc/source/admin/image-building.rst

@@ -74,6 +74,10 @@ There are following distros available for building images:
 - rhel
 - ubuntu
 
+See the :ref:`support matrix <support-matrix-base-images>` for information on
+supported base image distribution versions and supported images on each
+distribution.
+
 .. warning::
 
    Fedora images are deprecated since Newton and will be removed

doc/source/matrix_aarch64.csv

@@ -53,6 +53,7 @@ murano,C,C,C,C,N,C
 neutron,C,C,C,C,N,C
 neutron-mlnx-agent,C,C,N,C,N,C
 nova,C,C,C,C,N,C
+nova-spicehtml5proxy,N,N,C,C,N,C
 novajoin,C,C,N,C,N,C
 octavia,C,C,N,C,N,C
 opendaylight (deprecated),N,N,N,N,N,N

doc/source/matrix_x86.csv

@@ -13,6 +13,7 @@ collectd,C,C,C,C,N,C
 congress,C,C,N,C,N,C
 cron,T,T,T,T,N,T
 cyborg,N,C,N,N,N,N
+cyborg-agent,N,N,N,N,N,N
 designate,C,C,C,C,N,C
 dnsmasq,T,T,C,T,N,C
 ec2-api,C,C,N,C,N,C
@@ -24,6 +25,7 @@ glance,T,T,T,T,N,T
 gnocchi,C,C,C,C,N,C
 grafana,C,C,C,C,N,C
 hacluster,C,C,C,C,N,C
+hacluster-pcs,N,N,C,C,N,C
 haproxy,T,T,T,T,N,C
 heat,T,T,N,T,N,T
 horizon,T,T,T,T,N,T
@@ -53,6 +55,7 @@ murano,C,C,C,C,N,C
 neutron,T,T,T,T,N,T
 neutron-mlnx-agent,C,C,N,C,N,C
 nova,T,T,T,T,N,T
+nova-spicehtml5proxy,N,N,T,T,N,T
 novajoin,C,C,N,C,N,C
 octavia,C,C,N,C,N,C
 opendaylight (deprecated),C,C,C,C,N,N

doc/source/support_matrix.rst

@@ -4,6 +4,33 @@
 Kolla Images Support Matrix
 ===========================
 
+This page describes the supported base container image distributions and
+versions, and the Kolla images supported on each of those.
+
+.. _support-matrix-base-images:
+
+Supported base images
+=====================
+
+The following base container images are supported:
+
+.. note::
+
+   CentOS 7 is no longer supported as a base container image. The Train release
+   supports both CentOS 7 and 8 images, and provides a route for migration.
+
+============= ============ ================
+Distribution  Default base Default base tag
+============= ============ ================
+CentOS 8      centos       8
+Debian Buster debian       10
+RHEL 8        rhel         8
+Ubuntu Bionic ubuntu       18.04
+============= ============ ================
+
+The remainder of this document outlines which images are supported on which of
+these distribution.
+
 Support clause definitions
 ==========================
 

releasenotes/notes/centos-8-950d979507939643.yaml

@@ -0,0 +1,15 @@
+---
+features:
+  - |
+    Adds support for CentOS 8 as a base container image. This is the only major
+    version of CentOS supported from the Ussuri release. The Train release
+    supports both CentOS 7 and 8 images, and provides a route for migration.
+upgrade:
+  - |
+    CentOS 7 is no longer supported as a base container image. CentOS users
+    should migrate to CentOS 8. The Train release supports both CentOS 7 and 8
+    images, and provides a route for migration.
+  - |
+    The following images were supported by CentOS 7 but lack suitable packages
+    in CentOS 8, and are no longer supported for CentOS: ``cyborg-agent``,
+    ``hacluster-pcs``, ``nova-spicehtml5proxy``.