Grafana: update dashboards from git repo.

Currently, all grafana dashboards are created on Grafana host and might be lost if service loses relevant docker volume. It also impossible to easily replicate whole metrics service.

We want all Grafana dashboards to be initialized and later periodically updated from git repo snapshots. This will add proper versioning and backup for dashboards. Big benefit is being able to initialize whole service on dev machine with less manual work.

References:

https://grafana.com/docs/administration/provisioning/