Include pgbouncer by default in the pgconfig/jdbcconfig examples

[groldan]

Both the pgconfig and jdbcconfig examples store the Catalog and Config in a PostgreSQL database.

As the number of pods in a deployment increases, so do the chances of running out of server side connections.

At the geoserver-cloud side of the fence, we've tested using pgbouncer as a server-side connection pooler with several containers.

It'd be great if we can include such set up in the examples, especially due to the new support of auto-scaling.