Add loki datasource for grafana

flux/oas/loki-datasource_cm.yaml

+apiVersion: v1
+kind: ConfigMap
+metadata:
+  labels:
+    app: loki
+    grafana_datasource: "1"
+    release: loki
+  name: loki-datasource
+  namespace: oas
+data:
+  loki-stack-datasource.yaml: |-
+    apiVersion: 1
+    datasources:
+    - name: Loki
+      type: loki
+      access: proxy
+      url: http://loki:3100
+      version: 1