Use health endpoint for Grafana readiness probe (istio#16634)

Prior to this change we used the login page, which returns 25kb of data.
This will cause the probe to fail in Kubernetes 1.16+
(kubernetes/kubernetes#76518), which will limit
to 10kb.

This change mirrors the official Grafana charts readiness probe:
https://github.com/helm/charts/blob/master/stable/grafana/values.yaml#L18-L21

install/kubernetes/helm/istio/charts/grafana/templates/deployment.yaml

@@ -44,7 +44,7 @@ spec:
           - containerPort: 3000
           readinessProbe:
             httpGet:
-              path: /login
+              path: /api/health
               port: 3000
           env:
           - name: GRAFANA_PORT