Fix Grafana config file template to use variables

Grafana session/provider_config property actually uses a static user
and a static database name.
If grafana_database_user and grafana_database_name default value is
changed, the config generated will not be updated accordingly.

Change-Id: I517daab5439ab1aef56a4b3a11104bd0717223bf
Closes-Bug: #1682810

ansible/roles/grafana/templates/grafana.ini.j2

@@ -24,7 +24,7 @@ ssl_mode = disable
 
 [session]
 provider = mysql
-provider_config = grafana:{{ grafana_database_password }}@tcp({{ grafana_database_address }})/grafana
+provider_config = {{ grafana_database_user }}:{{ grafana_database_password }}@tcp({{ grafana_database_address }})/{{ grafana_database_name }}
 
 cookie_name = grafana_sess
 cookie_secure = false