* pgbackrest support
pgBackrest can restore in existing $PGDATA folder, this allows speedy restore as files which have not changed since last backup are skipped, to support this feature new param keep_data has been introduced. When keep_data=True, cleanup of $PGDATA will be skipped.
Patroni passes some extra parameters to custom_replica_methods when calling, this causes an error due to pgbackrest strict parameter checking. New param no_params=True can be set to skip parameters passing.
Fixes https://github.com/zalando/patroni/issues/625