Add bind mount for Grafana data

roles/prometheus/defaults/main.yml

@@ -1,3 +1,4 @@
 prom_name: prometheus
 grafana_name: grafana
 prom_root: "/opt/{{ prom_name }}"
+grafana_root: "/opt/{{ prom_name }}/grafana"

roles/prometheus/tasks/main.yml

@@ -53,6 +53,7 @@
     image: grafana/grafana:{{ grafana_version }}
     state: started
     restart_policy: always
+    volumes: "{{ grafana_root }}:/var/lib/grafana"
     networks_cli_compatible: true
     networks:
       - name: "{{ prom_name }}"