K8SAutoscale InCluster Configuration Problem

I'm currently not able to get the `k8sAutoscale()` node to work. When the `TICK` script is enabled, I get an error returned.

```
cannot use the k8sAutoscale node, could not create kubernetes client: unknown kubernetes cluster ""
```

I've configured `kapacitord` using environment variables. With my kapacitor docker container launched within my Kubernetes cluster. I verified the environment variables exist.

```
printenv | grep KAPACITOR_KUBERNETES
KAPACITOR_KUBERNETES_ENABLED=true
KAPACITOR_KUBERNETES_IN_CLUSTER=true
```

I tried to find the initializer for the In-Cluster configuration and found this line. Should this reference the `os.Getenv("KUBERNETES_SERVICE_HOST")` instead of having `http://kubernetes` hardcoded in as the url?

https://github.com/influxdata/kapacitor/blob/master/services/k8s/client/client.go#L95

*Version*
Using official `kapacitor:1.3` image

```
kapacitor version
Kapacitor 1.3.3 (git: HEAD ce586f35e89e75a1779e2b493caba15d66295a15)
```

*Environment*
Running inside Kubernetes 1.7.x within AWS

Start in services/k8s/client/client.go at the initializer around line 95, then reproduce the k8sAutoscale error with KAPACITOR_KUBERNETES_ENABLED, KAPACITOR_KUBERNETES_IN_CLUSTER, and the Kubernetes environment variables set. Done means the in-cluster Kubernetes client initializes without the unknown cluster error and the k8sAutoscale node can run.

Written by the indexing model from the issue text.