run postgres_extensible query across multiple databases

## Feature Request

Opening a feature request kicks off a discussion.

### Proposal:
The postgres_extensible plugin should support a way of running the same query across multiple databases. The configuration should allow running across all discovered databases, or a filtered list.

### Current behavior:
No such feature

### Desired behavior:
Such a feature

### Use case: [Why is this important (helps with prioritizing requests)]
Postgres provides a lot of data though what it calls ["system catalogs"](https://www.postgresql.org/docs/9.3/static/catalogs.html), which are really tables within each database. Thus to use them you have to connect to that database. Instead of having to know which databases are present, and then having to hard-code those databases in the telegraf config on each server, it would be nice if we could push out the same telegraf config to all servers.

Start by locating the postgres_extensible plugin and its configuration and database-discovery entry points. Clarify how all discovered databases and filtered database lists should be configured, then define tests that demonstrate the same query running across the selected databases.

Written by the indexing model from the issue text.