[pgpool] Unauthenticated access to postgres through pgpool

The bitnami/pgpool Docker image, and the bitnami/postgres-ha k8s chart, under default configurations, comes with an 'repmgr' user that allows unauthenticated access to the database inside the cluster. The PGPOOL_SR_CHECK_USER is the user that Pgpool itself uses to perform streaming replication checks against nodes, and should not be at trust level. This allows to log into a PostgreSQL database using the repgmr user without authentication. If Pgpool is exposed externally, a potential attacker could use this user to get access to the service. This is also present within the bitnami/postgres-ha Kubernetes Helm chart.