[fluentd] add K8S_NODE_NAME env variable for Kubernetes metadata plugin

[js-timbirkett]

Adds the K8S_NODE_NAME env variable as suggested here: https://github.com/fabric8io/fluent-plugin-kubernetes_metadata_filter#environment-variables-for-kubernetes

[mebays]

Is there any idea when this will get merged?

[KeenanLawrence]

I don't understand why this hasn't been merged yet. If the application throws a warning and this reduces cache misses and needless calls to the Kubernetes API, then it should be provided by default.