Always generate grafana-config secret

Since https://github.com/brancz/kubernetes-grafana/pull/115, upstream
grafana contains a non-empty config. Generate the grafana-config secret
unconditionally even if no user config is passed.

manifests/grafana-config.yaml

@@ -0,0 +1,15 @@
+apiVersion: v1
+kind: Secret
+metadata:
+  labels:
+    app.kubernetes.io/component: grafana
+    app.kubernetes.io/name: grafana
+    app.kubernetes.io/part-of: kube-prometheus
+    app.kubernetes.io/version: 8.1.3
+  name: grafana-config
+  namespace: monitoring
+stringData:
+  grafana.ini: |
+    [date_formats]
+    default_timezone = UTC
+type: Opaque