Cleanup and unify postgres dashboards

The Grafana dashboards for postgres are inconsistent in their use of variables and have hardcoded values for environment, type and prometheus instances in many places, which makes it hard to adapt them for new clusters or envs (like for patroni).

We should update the dashboards to consistently use selectable variables for environment and add a new variable type to switch between "patroni", "postgres" and possibly other db host types in the future. That would make the dashboards re-usable when adding patroni to production.