Reduce wal_retrieve_retry_interval in applicable TAP tests.

By default, wal_retrieve_retry_interval is five seconds, which is far
more than is needed in any of our TAP tests, leaving the test cases
just twiddling their thumbs for significant stretches.  Moreover,
because it's so large, we get basically no testing of the retry-before-
master-is-ready code path.  Hence, make PostgresNode::init set up
wal_retrieve_retry_interval = '500ms' as part of its customization of
test clusters' postgresql.conf.  This shaves quite a few seconds off
the runtime of the recovery TAP tests.

Back-patch into 9.6.  We have wal_retrieve_retry_interval in 9.5,
but the test infrastructure isn't there.

Discussion: https://postgr.es/m/31624.1498500416@sss.pgh.pa.us