Fix grafana post-config check

Check grafana's /login url for status code 200 instead of root url since the
latter does not return an http 200 code

Change-Id: Ide7da26c0e217a4a60ad3d2ac693d6f7ac7bb722
Closes-Bug: #1705911

ansible/roles/grafana/tasks/post_config.yml

 ---
 - name: Wait for grafana application ready
   uri:
-    url: "{{ internal_protocol }}://{{ kolla_internal_vip_address }}:{{ grafana_server_port }}"
+    url: "{{ internal_protocol }}://{{ kolla_internal_vip_address }}:{{ grafana_server_port }}/login"
     status_code: 200
   register: result
   until: result.status == 200