nodepool: pause opensuse-15 builds

These builds are constantly failing (logs at [1], [2]) so pending
investigation, pause the builds to avoid filling the builders.

[1] https://nb01.opendev.org/
[2] https://nb02.opendev.org/

Change-Id: Ic280568c0c084f112cf496178456b898b8cdc18b

nodepool/nodepool.yaml

@@ -246,7 +246,9 @@ diskimages:
 
   - name: opensuse-15
     parent: base
-    pause: false
+    # NOTE(ianw): Paused 2020-12-18 due to persistent build failures
+    # pending investigation
+    pause: true
     elements:
       - opensuse-minimal
       - cache-devstack