Implement centralized logging

Fix #263

roles/promtail/tasks/main.yml

+---
+- name: install promtail
+  pacman: name=promtail state=present
+
+- name: install promtail configuration
+  template: src=promtail.yaml.j2 dest=/etc/loki/promtail.yaml owner=root group=promtail mode=0640
+  notify: restart promtail
+
+- name: open promtail ipv4 port for monitoring.archlinux.org
+  ansible.posix.firewalld: state=enabled permanent=true immediate=yes
+    rich_rule="rule family=ipv4 source address={{ hostvars['monitoring.archlinux.org']['ipv4_address'] }} port protocol=tcp port=9080 accept"
+  tags:
+    - firewall
+
+- name: start and enable promtail
+  systemd: name=promtail.service enabled=yes daemon_reload=yes state=started

roles/promtail/templates/promtail.yaml.j2

+server:
+  http_listen_address: 127.0.0.1
+  http_listen_port: 9080
+  grpc_listen_address: 127.0.0.1
+  grpc_listen_port: 0 # 0 means random
+
+positions:
+  filename: /var/lib/promtail/positions.yaml
+
+clients:
+  - url: https://{{ logging_domain }}/loki/api/v1/push
+    basic_auth:
+      username: '{{ vault_loki_nginx_user }}'
+      password: '{{ vault_loki_nginx_passwd }}'
+
+scrape_configs:
+  - job_name: journal
+    journal:
+      json: true
+      max_age: 72h
+      path: /var/log/journal
+      labels:
+        job: systemd-journal
+    relabel_configs:
+      - source_labels: ['__journal__systemd_unit']
+        target_label: unit
+      - source_labels: ["__journal__hostname"]
+        target_label: instance