Adds "domain-servers-list" to test_domains.py
Adds "domain-servers-list" to test_domains.py

This submission adds a unittest "test_list_servers_for_a_domain"
under "designate/tests/test_api/test_v1/test_domains.py" script
to validate "domain-servers-list" functionality.

Change-Id: Iec31efaa0c30829351d5eb04e419f9ace30537be

tonyli71/designate,cneill/designate-testing,muraliselva10/designate,cneill/designate,tonyli71/designate,cneill/designate-testing,tonyli71/designate,kiall/designate-py3,grahamhayes/designate,cneill/designate-testing,ramsateesh/designate,openstack/designate,grahamhayes/designate,kiall/designate-py3,ramsateesh/designate,ionrock/designate,kiall/designate-py3,cneill/designate,cneill/designate,cneill/designate,ionrock/designate,kiall/designate-py3,openstack/designate,openstack/designate,grahamhayes/designate,ionrock/designate,muraliselva10/designate,cneill/designate,ramsateesh/designate,kiall/designate-py3,muraliselva10/designate