Change centos DIB_RELEASE default to 9-stream

Since the public mirrors for centos8 are really not available
anymore since it has gone End of Life, changes the default
invocation for a quick disk image to 9-stream.

Users who wish specific behavior should explicitly express
the version they desire.

Change-Id: I12cb10258ab80bae1c754e86aadcf926cdfae805

diskimage_builder/elements/centos/README.rst

@@ -24,3 +24,10 @@ DIB_CLOUD_IMAGES:
                 ppc64le images. Until then you'll need to set this to a local
                 image file.
   :Example: ``DIB_CLOUD_IMAGES=/path/to/my/centos/8/CentOS-8-GenericCloud-x86_64.qcow2``
+
+DIB_RELEASE:
+  :Required: No
+  :Description: To override the release to build, utilize this paraemter.
+                Presently defaults to ``9-stream``, however may be changed by
+                diskimage-builder to newer versions *as* the distribution
+                versions move through their lifecycle.

diskimage_builder/elements/centos/environment.d/10-centos-distro-name.bash

@@ -1,3 +1,3 @@
 export DISTRO_NAME=centos
-export DIB_RELEASE=${DIB_RELEASE:-8}
+export DIB_RELEASE=${DIB_RELEASE:-9-stream}
 export EFI_BOOT_DIR="EFI/centos"

releasenotes/notes/centos-default-change-to-9-stream-45a0a88db60d7469.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    The default release for the ``centos`` element is now "9-stream" instead of
+    "8". This is a result of Centos8 having reached End of Life.