loki: Add role+playbook for Grafana Loki

The current Grafana Loki server, *loki0.pyrocufflink.blue*, runs Fedora
CoreOS and is managed by Ignition and *cfg*.  Since I have declared
*cfg* a failed experiment, I'm going to re-deploy Loki on a new VM
running Fedora Linux and managed by Ansible.

The *loki* role installs Podman and defines a systemd-managed container
to run Grafana Loki.

roles/loki/defaults/main.yml

@@ -0,0 +1,39 @@
+loki_config:
+  auth_enabled: false
+
+  server:
+    http_listen_port: 3100
+    http_listen_address: 127.0.0.1
+    grpc_listen_port: 9096
+
+  common:
+    instance_addr: 127.0.0.1
+    path_prefix: /var/lib/loki
+    storage:
+      filesystem:
+        chunks_directory: /var/lib/loki/chunks
+        rules_directory: /var/lib/loki/rules
+    replication_factor: 1
+    ring:
+      kvstore:
+        store: inmemory
+
+  query_range:
+    results_cache:
+      cache:
+        embedded_cache:
+          enabled: true
+          max_size_mb: 100
+
+  schema_config:
+    configs:
+    - from: 2020-10-24
+      store: tsdb
+      object_store: filesystem
+      schema: v12
+      index:
+        prefix: index_
+        period: 24h
+
+  query_scheduler:
+    max_outstanding_requests_per_tenant: 1024