Force osd pool min_size value to integer

Force osd pool min_size value to integer

After b8d580b and e9e5d5a we could have either item.min_size or
osd_pool_default_min_size using string instead of int causing the
condition to be true when it's false.
As a result, the task could try to set the pool min_size value to
0 which leads to:

Error EINVAL: pool min_size must be between 1 and 1

Signed-off-by: Dimitri Savineau <dsavinea@redhat.com>