Update grafana to its latest version (#7945)
Update grafana to its latest version (#7945)

* Update grafana to its latest version

https://github.com/istio/istio/blob/master/addons/grafana/Dockerfile.grafana#L1 shows
that Istio currently uses the version 5.0.4 of grafana, which was released on March 27, 2018.
The latest version of grafana is 5.2.2, released on July 24, 2018. Istio should update its
grafana version to leverage the fixes included in the latest grafana version.

* Fix the error on "make docker.grafana"

istio/istio,geeknoid/istio,smawson/istio,geeknoid/istio,smawson/istio,smawson/istio,istio/istio,istio/istio,geeknoid/istio,geeknoid/istio,smawson/istio,geeknoid/istio,geeknoid/istio,smawson/istio,istio/istio,istio/istio,smawson/istio