Add an option to assign machines by default to a pool that is not the 'default' one

Currently maas only adds the new machines to the ‘default’ pool, as described here: https://maas.io/docs/how-to-manage-machines#heading--how-to-use-resource-pools

A nice feature will be to be able to change the ‘default’ pool with another pool, so that newly added machines can be managed by non-admin users via RBAC.