ceph-defaults: pin grafana container tag to 5.2.4

The latest grafana container tag is using grafana 6.x release which could
cause issue with the ceph dashboard integration.
Considering that the grafana container in RHCS 3 is based on 5.x then we
should use the same version.

$ docker run --rm rhceph/rhceph-3-dashboard-rhel7:3 -v
Version 5.2.4 (commit: unknown-dev)
Signed-off-by: Dimitri Savineau <dsavinea@redhat.com>

group_vars/all.yml.sample

@@ -724,7 +724,7 @@ dummy:
 # We only need this for SSL (https) connections
 #grafana_crt: ''
 #grafana_key: ''
-#grafana_container_image: "grafana/grafana:latest"
+#grafana_container_image: "grafana/grafana:5.2.4"
 #grafana_container_cpu_period: 100000
 #grafana_container_cpu_cores: 2
 # container_memory is in GB

roles/ceph-defaults/defaults/main.yml

@@ -716,7 +716,7 @@ grafana_admin_password: admin
 # We only need this for SSL (https) connections
 grafana_crt: ''
 grafana_key: ''
-grafana_container_image: "grafana/grafana:latest"
+grafana_container_image: "grafana/grafana:5.2.4"
 grafana_container_cpu_period: 100000
 grafana_container_cpu_cores: 2
 # container_memory is in GB