Gate: Consolidate job definition to allow imports

This PS moves the Zuulv3 infra jobs to use a base that the various
nodesets can inherit from.

Change-Id: I8f9380d00d874d102d4a5c977edad499872006a9

.zuul.yaml

@@ -80,19 +80,21 @@
           - node-1
           - node-2
 - job:
-    name: openstack-helm-infra-ubuntu
+    name: openstack-helm-infra
     pre-run: tools/gate/playbooks/zuul-pre.yaml
     run: tools/gate/playbooks/zuul-run.yaml
+
+- job:
+    name: openstack-helm-infra-ubuntu
+    parent: openstack-helm-infra
     nodeset: openstack-helm-ubuntu
 
 - job:
     name: openstack-helm-infra-centos
-    pre-run: tools/gate/playbooks/zuul-pre.yaml
-    run: tools/gate/playbooks/zuul-run.yaml
+    parent: openstack-helm-infra
     nodeset: openstack-helm-centos
 
 - job:
     name: openstack-helm-infra-fedora
-    pre-run: tools/gate/playbooks/zuul-pre.yaml
-    run: tools/gate/playbooks/zuul-run.yaml
+    parent: openstack-helm-infra
     nodeset: openstack-helm-fedora