Default file-name for database path for sqlite3

<!-- Please only use this template for submitting feature requests --> When you are using the sqlite3 database and want to store it in a specific folder, you use the database path parameter. On https://grafana.com/docs/grafana/latest/setup-grafana/configure-grafana/#path this parameter has the following description: "Only applicable for sqlite3 database. The path to the database file." Unfortunately the content of the path-parameter must not be a folder, but needs to be a file. Which you can name anything you want and which doesn't need to be there yet. Grafana will create the file itself. If the parameter doesn't contain a file-name, Grafana goes into errors. **What would you like to be added**: It might be an idea to let Grafana use a default-name when the path-parameter doesn't contain a file-name. **Who is this feature for?** This feature is for people installing Grafana who wants to store the Grafana-database on storage rather than in a database. When installing on Kubernetes only the last lines of the pod's log are visible. So I missed the message that the path-parameter (GF_DATABASE_PATH) was referring to a folder. It was quite a puzzle to find out that this was the case.