On Debian/Ubuntu systems it is common to keep Postgres config files outside of the data directory.
It created a couple of problems for pg_rewind support in Patroni.
1. The `--config_file` argument must be supplied while figuring out the `restore_command` GUC value on Postgres v12+
2. With Postgres v13+ pg_rewind by itself can't find postgresql.conf in order to figure out `restore_command` and therefore we have to use Patroni as a fallback for fetching missing WAL's that are required for rewind.
This commit addresses both problems.