Revert "Switch images to CentOS Stream 8"

This reverts commit
0d16bee11b.

It is being reverted as the centos stream images
contain extra, un-necessary libraries and packages
installed which swells the ramdisk size up substantially
and is causing failures in CI as the compressed image size
expanded by about 100MB, and uncompressed the stream images
are 1.1GB.

Change-Id: I71eed30c27c109ac03d41e6ec46067148eb9b007

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

@@ -1,4 +0,0 @@
----
-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 Stream 8
+- name: Default to CentOS 8
   set_fact:
-    image_release: 8-stream
+    image_release: 8
   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 | replace('-stream', '') }}
+    image_target_name: centos{{ image_release }}
   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-stream
+      image_release: 8
 
 - job:
     name: ironic-python-agent-check-image-base
@@ -95,7 +95,7 @@
     vars:
       image_type: 'dib'
       image_distro: 'centos'
-      image_release: 8-stream
+      image_release: 8
 
 - job:
     name: ironic-python-agent-check-image-dib-centos8-extra