Return PoolList as a right list object
Return PoolList as a right list object

- We are using PoolAttributeList object for this case
and put a list of Pool objects for it.

Co-authored-By: Nguyen Van Trung <trungnv@vn.fujitsu.com>

Change-Id: Ie9047c6623d5b54c1ef890f43cff759c1b537797

openstack/designate,openstack/designate,openstack/designate