ceph_pool: set target size ratio on both 'on' and 'warn' mode

ceph_pool: set target size ratio on both 'on' and 'warn' mode

when we set target_size_ratio to warn it means that the administrator wants to get suggestion from the mgr module but apply it manually when he/she wants. So it's in the same approach as 'on' mode just triggered by hand.
So there is no need to set pg_num when target_size_ratio is 'warn' and the mgr module will calculate the correct pg_num and the administrator will adjust it whenever he/she wants.

It is the same approach that was in #6471

Signed-off-by: Seena Fallah <seenafallah@gmail.com>