mgr/pg_autoscaler: treat target ratios as weights

mgr/pg_autoscaler: treat target ratios as weights

Normalize across pools so that it's simpler to use - this way you
don't have to adjust every other pool when you add one.

Handle pools with target_bytes by taking their capacity off the top,
and dividing the rest into the pools with a target_ratio.

If both target bytes and ratio are specified, ignore bytes. This
matches the docs and makes accounting simpler.

Fixes: https://tracker.ceph.com/issues/43947
Signed-off-by: Josh Durgin <jdurgin@redhat.com>