mgr/pg_autoscaler: avoid scale-down until there is pressure

mgr/pg_autoscaler: avoid scale-down until there is pressure

The autoscaler will start out with scaling each
pools to have a full complements of pgs from the start
and will only decrease it when pools need more due to
increased usage.

Introduced a unit test that tests only the
function get_final_pg_target_and_ratio() which
deals with the distrubtion of pgs amongst the
pools

Edited workunit script to reflect the change
of how pgs are calculated and distrubted.

Signed-off-by: Kamoltat <ksirivad@redhat.com>
(cherry picked from commit daeb6f6ac0c8f77ae07147f9d1e2ed18d6d8e4cc)