azure: enable testing with LTS on AKS

Author: nojnhuh

cluster-autoscaler/cloudprovider/azure/test/templates/cluster-template-prow-aks-aso-cluster-autoscaler.yaml

@@ -49,6 +49,10 @@ spec:
           enabled: true
       servicePrincipalProfile:
         clientId: msi
+      sku:
+        name: Base
+        tier: ${AKS_TIER:=Standard}
+      supportPlan: ${AKS_SUPPORT_PLAN:=KubernetesOfficial}
       tags:
         buildProvenance: ${BUILD_PROVENANCE}
         creationTimestamp: ${TIMESTAMP}