ceph-rgw: Fix custom pool size setting

ceph-rgw: Fix custom pool size setting

RadosGW pools can be created by setting

```yaml
rgw_create_pools:
  .rgw.root:
    pg_num: 512
    size: 2
```

for instance. However, doing so would create pools of size
`osd_pool_default_size` regardless of the `size` value. This was due to
the fact that the Ansible task used

```
{{ item.size | default(osd_pool_default_size) }}
```

as the pool size value, but `item.size` is always undefined; the
correct variable is `item.value.size`.

Signed-off-by: Benoît Knecht <bknecht@protonmail.ch>