This adds INFO log messages that clearly state if configuration values were seen as changed by Patroni after SIGHUP/reload and warrant reloading (or if nothing was changed an no reloading is necessary).
This ended up being a lot simpler than I had imagined once I found postgresql.py:reload_config().
I add a log line in config.py:reload_local_configuration() since that function will short-circuit the process early if the local config wasn't changed. But the final determination of whether or not values have changed and need reloading is in postgresql.py:reload_config().
Add an EnvironmentFile directive to read in a configuration file with environment variables. The "-" prefix means it can proceed if the file doesn't exist.
This would allow users to keep sensitive information like the SUPERUSER/REPLICATION passwords in the config file separate from a YAML file that might be deployed from source control.
Fix the discrepancy for the values of max_wal_senders and max_replication_slots between the sample postgres.yml files and hard-coded defaults in Patroni, bumping the former to 10.
Contributed by @dtseiler
