promtail: Role/Playbook to deploy Promtail

Promtail is the log sending client for Grafana Loki.  For traditional
Linux systems, an RPM package is available from upstream, making
installation fairly simple.  Configuration is stored in a YAML file, so
again, it's straightforward to configure via Ansible variables.  Really,
the only interesting step is adding the _promtail_ user, which is
created by the RPM package, to the _systemd-journal_ group, so that
Promtail can read the systemd journal files.

roles/promtail/vars/main.yml

@@ -0,0 +1,29 @@
+promtail_default_scrape:
+  journal:
+    job_name: journal
+    journal:
+      json: false
+      labels:
+        job: systemd-journal
+    relabel_configs:
+      - source_labels:
+          - __journal__hostname
+        target_label: hostname
+      - source_labels:
+          - __journal__systemd_unit
+        target_label: unit
+      - source_labels:
+          - __journal_syslog_identifier
+        target_label: syslog_identifier
+      - source_labels:
+          - __journal_priority
+        target_label: priority
+      - source_labels:
+          - __journal_message_id
+        target_label: message_id
+      - source_labels:
+          - __journal__comm
+        target_label: command
+      - source_labels:
+          - __journal__transport
+        target_label: transport