Add real default value for osd pool size customization.
Ceph itself has an `osd_pool_default_size` default value to `3`.

If users don't specify a pool size in various pools definition within
ceph-ansible, we should default to `3`.

By the way, this kind of condition isn't really clear:
```
when:
  - rbd_pool_size | default ("")
```

we should try to get the customized value then default to what is in
`osd_pool_default_size` (which has its default value pointing to
`ceph_osd_pool_default_size` (`3`) as well) and compare it to
`ceph_osd_pool_default_size`.

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>
(cherry picked from commit 7774069d)