nova-multi-cell: Skip test_cold_migrate_unshelved_instance
nova-multi-cell: Skip test_cold_migrate_unshelved_instance

Change-Id: I494bf6b10ffb2f54ac4d6079760b44178d82b3fd
Related-Bug: #1906428

klmitch/nova,mahak/nova,klmitch/nova,mahak/nova,klmitch/nova,mahak/nova,openstack/nova,openstack/nova,klmitch/nova,openstack/nova