Switch images to CentOS Stream 8

Regular CentOS 8 is deprecated and is planned to reach EOL in 2021.
Use CentOS Stream which will also give us access to bug fixes and
features earlier.

Change-Id: I485271ade7594f7c4d17ed81aa677679ef4fb49e

releasenotes/notes/centos8-stream-55d4100dcf3aaa8f.yaml

@@ -0,0 +1,4 @@
+---
+upgrade:
+  - |
+    Pre-built images are now built with CentOS Stream 8 instead of just CentOS.

roles/ipa-build-dib-image/tasks/main.yaml

@@ -1,13 +1,13 @@
-- name: Default to CentOS 8
+- name: Default to CentOS Stream 8
   set_fact:
-    image_release: 8
+    image_release: 8-stream
   when:
     - image_distro in ["centos-minimal", "centos"]
     - not image_release
 
 - name: Generate a nice target name for CentOS
   set_fact:
-    image_target_name: centos{{ image_release }}
+    image_target_name: centos{{ image_release | replace('-stream', '') }}
   when:
     - image_distro in ["centos-minimal", "centos"]
     - not image_target_name

zuul.d/ironic-python-agent-builder-jobs.yaml

@@ -37,7 +37,7 @@
     vars:
       image_type: 'dib'
       image_distro: 'centos'
-      image_release: 8
+      image_release: 8-stream
 
 - job:
     name: ironic-python-agent-check-image-base
@@ -95,7 +95,7 @@
     vars:
       image_type: 'dib'
       image_distro: 'centos'
-      image_release: 8
+      image_release: 8-stream
 
 - job:
     name: ironic-python-agent-check-image-dib-centos8-extra