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/log-collector/defaults/main.yml

@@ -2,6 +2,7 @@
 
 # Whether to set up elasticsearch/kibana at all.
 enable_elasticsearch: true
+enable_loki: false
 
 es_major_version: "6"