Fix Grafana config file template to use variables
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

dardelean/kolla-ansible,dardelean/kolla-ansible,dardelean/kolla-ansible