Fix the nodeset "devstack-single-node-opensuse-15"

The label "opensuse-15" is no longer available since [1]. Since there
are pending references to this nodeset from older branches, use an
empty node list for it to fix the zuul config until all references
can be dropped.

[1]https://review.opendev.org/c/openstack/project-config/+/955214

Signed-off-by: Rodolfo Alonso Hernandez <ralonsoh@redhat.com>
Change-Id: I2f5105178482402aa108910d1bd1ec2f2c7c8933

.zuul.yaml

@@ -48,15 +48,10 @@
         nodes:
           - controller
 
+# TODO(frickler): drop this dummy nodeset once all references have been removed
 - nodeset:
     name: devstack-single-node-opensuse-15
-    nodes:
-      - name: controller
-        label: opensuse-15
-    groups:
-      - name: tempest
-        nodes:
-          - controller
+    nodes: []
 
 - nodeset:
     name: devstack-single-node-debian-bookworm