Switch to CentOS 8 Stream for testing

Regular CentOS 8 has been deprecated and is supposed to reach EOL
in the end of 2021. This change switches to CentOS Stream.
The documentation is updated to explain the current status.

Change-Id: I8af7ff0069bee2b2485d6f67f4f5a68631099f22

doc/source/install/index.rst

@@ -15,8 +15,7 @@ Requirements
 Supported operating systems:
 
 * Ubuntu 18.04, 20.04
-* Red Hat Enterprise Linux (RHEL) 8
-* CentOS 8
+* CentOS 8 Stream (normal CentOS 8 and RHEL 8 should work but are not tested)
 * openSUSE Leap 15.2 (15.1 is supported but not recommended)
 * Fedora 32 (30 is supported but not recommended)
 * Debian Buster

releasenotes/notes/centos8-stream-0c6d9adb544e36af.yaml

@@ -0,0 +1,8 @@
+---
+upgrade:
+  - |
+    Following an `announcement
+    <https://lists.centos.org/pipermail/centos-announce/2020-December/048208.html>`_
+    by the CentOS project, Bifrost has switched to CentOS Stream for testing.
+    Regular CentOS is no longer tested in the CI, meaning that both it and
+    RHEL will only be tested indirectly and supported on the best effort basis.

zuul.d/bifrost-jobs.yaml

@@ -44,7 +44,7 @@
 - job:
     name: bifrost-integration-dhcp-centos-8
     parent: bifrost-integration-dhcp
-    nodeset: centos-8
+    nodeset: centos-8-stream
 
 - job:
     name: bifrost-integration-dibipa-debian
@@ -61,7 +61,7 @@
 - job:
     name: bifrost-integration-dibipa-debian-centos-8
     parent: bifrost-integration-dibipa-debian
-    nodeset: centos-8
+    nodeset: centos-8-stream
 
 - job:
     name: bifrost-integration-tinyipa
@@ -126,12 +126,12 @@
 - job:
     name: bifrost-integration-tinyipa-centos-8
     parent: bifrost-integration-tinyipa
-    nodeset: centos-8
+    nodeset: centos-8-stream
 
 - job:
     name: bifrost-integration-tinyipa-keystone-centos-8
     parent: bifrost-integration-tinyipa
-    nodeset: centos-8
+    nodeset: centos-8-stream
     required-projects:
       - openstack/keystone
       - openstack/ironic-prometheus-exporter