Deprecate send_cluster_metrics

Currently, Magnum is running periodic tasks to collect k8s cluster metrics to message bus. Unfortunately, it's collecting pods info only from "default" namespace which makes this function useless. What's more, even Magnum can get all pods from all namespaces, it doesn't make much sense to keep this function in Magnum. Because operators only care about the health of cluster nodes. If they want to know the status of pods, they can use heapster or other tools to get that.