Errors when running imagenet sample (kubeflow#761)

* Typo in README.txt INGRESS_GATEWAY is a variable

* Namespace for samples cannot be default namespace

When I apply the imagenet.yaml (kubectl create -f imagenet.yaml), I get the error 
Error from server: error when creating "imagenet.yaml": admission webhook "inferenceservice.kfserving-webhook-server.validator" denied the request: Cannot create the Inferenceservice "imagenet" in namespace "default": the namespace lacks label "serving.kubeflow.org/inferenceservice: enabled"


I guess that the inferenceservice must be applied in the <user_namespace> (as the other samples)

docs/samples/explanation/alibi/imagenet/README.md

@@ -46,7 +46,7 @@ Set up some environment variables for the model name and cluster entrypoint. Use
 
 ```
 INGRESS_GATEWAY=istio-ingressgateway
-export CLUSTER_IP=$(kubectl -n istio-system get service INGRESS_GATEWAY -o jsonpath='{.status.loadBalancer.ingress[0].ip}')
+export CLUSTER_IP=$(kubectl -n istio-system get service $INGRESS_GATEWAY -o jsonpath='{.status.loadBalancer.ingress[0].ip}')
 ```
 
 Test the predictor on an example image:

docs/samples/explanation/alibi/imagenet/imagenet.yaml

@@ -2,7 +2,6 @@ apiVersion: "serving.kubeflow.org/v1alpha2"
 kind: "InferenceService"
 metadata:
   name: "imagenet"
-  namespace: default
 spec:
   default:
     predictor: