ceph-pool: pass pg_num for pg_autoscale_mode warn

ceph-pool: pass pg_num for pg_autoscale_mode warn

The pg_autoscale_mode parameter has three possible values 'on', 'off', and
'warn'. In case of 'warn' it should be possible to pass the number of
placement groups (pg_num) to the create_pool function. Nothing is lost by
allowing this. The health check will still warn if the value needs
adjustment.

The evaluation of target_size_ratio works as before.

Add a test iterating through possible value combinations of the parameters
target_size_ratio, pg_num and pg_autoscale_mode.

Signed-off-by: Christian Brandt <cbrandt@strato.de>