Follow best practices for metadata plugin

As discussed in #16, it's recommended to set an env var K8S_NODE_NAME:
https://github.com/fabric8io/fluent-plugin-kubernetes_metadata_filter#environment-variables-for-kubernetes

Author: jvassev

log-router/Chart.yaml

@@ -4,4 +4,4 @@
 apiVersion: v1
 description: Distribution of Fluentd as K8S daemonset
 name: log-router
-version: 0.2.1
+version: 0.2.2

log-router/templates/daemonset.yaml

@@ -26,6 +26,11 @@ spec:
       containers:
         - name: fluentd
           image: "{{ .Values.image.repository }}:{{ .Values.image.tag }}"
+          env:
+          - name: K8S_NODE_NAME
+            valueFrom:
+              fieldRef:
+                fieldPath: spec.nodeName
           imagePullPolicy: {{ .Values.image.pullPolicy }}
           volumeMounts:
           - name: fluentconf