disable managed identity in test config

Author: andyzhangx

test/manifest/linux.json

@@ -6,6 +6,7 @@
             "orchestratorType": "Kubernetes",
             "orchestratorRelease": "1.16",
             "kubernetesConfig": {
+                "useManagedIdentity": false,
                 "useCloudControllerManager": true,
                 "customCcmImage": "mcr.microsoft.com/k8s/core/azure-cloud-controller-manager:v0.3.0",
                 "networkPlugin": "azure",