Draft: Enable Loki as a replacement for Elasticsearch when `enable_loki` is set.

Note, it is not possible to have both Loki and Elasticsearch enabled, so enable_elasticsearch must be disabled, when enable_loki is enabled.

roles/prometheus/files/grafana/provisioning/datasources/loki.yaml

+apiVersion: 1
+datasources:
+  - name: Loki
+    type: loki
+    access: proxy
+    url: http://localhost:3100
+    jsonData:
+      maxLines: 1000