Stop building CentOS 8

In line with CentOS deprecation, http://mirrorlist.centos.org has
stopped returning mirror sources.  As the dib builds have this in the
initial image clones, they won't build any more.  Pause this to stop a
spiral of failing builds.

Since we setup local mirrors, things might keep working in our
ecosystem for a bit, so far the upstream mirrors haven't pulled the 8/
directory directly it seems.

Change-Id: I99ca38f346b3a5d19d7565c1f6114a51b7fe2da3

nodepool/nb03.opendev.org.yaml

@@ -150,6 +150,7 @@ diskimages:
       DIB_DISTRIBUTION_MIRROR: 'https://mirror.regionone.linaro-us.opendev.org/debian'
 
   - name: centos-8-arm64
+    pause: true
     parent: base
     python-path: /usr/bin/python3
     elements:

nodepool/nodepool.yaml

@@ -151,7 +151,7 @@ diskimages:
 
   - name: centos-8
     parent: base
-    pause: false
+    pause: true
     python-path: /usr/bin/python3
     release: '8'
     elements: