qa: use the pg autoscale mode to calcuate the pg_num

qa: use the pg autoscale mode to calcuate the pg_num

Setting the pg_num to 8 is too small that some osds maybe not covered by the
pools, some osds maybe overloaded. Remove the hardcodeing pg_num here and let
the pg autoscale mode to calculate it as needed, and at the same time set the
pg_num_min to 64 to avoid the pg_num to small.

If ec pool is used, for the test cases most datas will go to the ec pool and
the primary replicated pool will store a small amount of metadata for all the
files only, so set the target size ratio to 0.05 should be enough.

Fixes: https://tracker.ceph.com/issues/45434
Signed-off-by: Xiubo Li <xiubli@redhat.com>