The buildfarm client uses TEMP_CONFIG to implement its extra_config
setting.  Except for stats_temp_directory, extra_config now applies to
TAP suites; extra_config values seen in the past month are compatible
with this.  Back-patch to 9.6, where PostgresNode was introduced, so the
buildfarm can rely on it sooner.

Reviewed by Andrew Dunstan and Tom Lane.

Discussion: https://postgr.es/m/20181229021950.GA3302966@rfd.leadboat.com