Draft: Clickhouse as log-collector backend

See issue #144

roles/float-infra-log-collector/defaults/main.yml

 ---
 
-# Whether to set up elasticsearch/kibana at all.
-enable_elasticsearch: true
+# Whether to set up Clickhouse.
+enable_clickhouse: true
 
-es_major_version: "6"
+clickhouse_username: clickhouse
 
-# This number is very low and only useful for the testing environment.
-es_heap_size: "166m"
-
-# Each member of the following dictionary is the prefix of a logstash-like
-# timestamped index. The value is how many days of logs to keep.
-es_log_keep_days:
-  audit: 60
-  logstash: 15
-  http: 15
-
-kibana_default_index_id: "ec2f3610-55c7-11e8-823a-73397be2582f"