qa: descrease pgbench scale factor to 32 for postgresql database test

qa: descrease pgbench scale factor to 32 for postgresql database test

The scale factor will depend on the node's performance and disk sizes
being used to run the test, and 500 seems too large here as a common
test case without knowing the nodes' configuration.

We should just follow the white-book:
https://github.com/ApsaraDB/PolarDB-for-PostgreSQL/blob/distributed/doc/polardb/benchmark.md

Fixes: https://tracker.ceph.com/issues/62700
Signed-off-by: Xiubo Li <xiubli@redhat.com>
(cherry picked from commit 497f951c0ddcb4be3921ef887f866eb534258537)