nodepool: pause centos-8-stream builds

A new release of rng-tools is causing the build to constantly time-out
[1].  I'm not confident with the dib change until it's had more review
[2], and it will take us some time to release or a fixed package to
come through anyway.  In the mean-time, pause the build.

[1] https://bugzilla.redhat.com/show_bug.cgi?id=1975588
[2] https://review.opendev.org/c/openstack/diskimage-builder/+/797794

Change-Id: Ieabc9d9b51e0a908480dfb56cfc5baf0d0760365

nodepool/nodepool.yaml

@@ -163,7 +163,7 @@ diskimages:
 
   - name: centos-8-stream
     parent: base
-    pause: false
+    pause: true
     python-path: /usr/bin/python3
     release: '8-stream'
     elements: