Simple curl service

This sample is a a request source for invoking other services, to experiment with Istio networking. It consists of a pod that does nothing but sleep. You can get a shell on the pod (an Alpine container) and use curl.

To use it:

Install Istio by following the installation instructions.
Start the curl pod:
```
kubectl apply -f curl.yaml
```

Start some other services, for example, the Bookinfo sample.

Now you can kubectl exec into the curl service to experiment with Istio networking. For example, the following commands can be used to call the Bookinfo ratings service:

export CURL_POD=$(kubectl get pod -l app=curl -o jsonpath={.items..metadata.name})
kubectl exec -it $CURL_POD -c curl -- curl http://ratings.default.svc.cluster.local:9080/ratings/1
{"id":1,"ratings":{"Reviewer1":5,"Reviewer2":4}}

You can also use the curl service to test accessing services outside of the mesh. See configuring egress for details.