Unbreak pgpool_setup and regression tests against pre-10 PostgreSQL.

Unbreak pgpool_setup and regression tests against pre-10 PostgreSQL.

Commit 26446126f36dcd34ea9032ac934aafe63acc0eee broke pgpool_setup and
regression tests against pre-10 PostgreSQL because they unconditionally
use feature introduced in PostgreSQL 10, i.e. SCRAM authentication. To
fix this, check PostgreSQL version using initdb -v and not use
PostgreSQL 10 specific feature if PostgreSQL 9.6 or earlier is used.
In this case regression tests 020, 021 and 022 always success.