Update readme to reflect defcore test list removal
Update readme to reflect defcore test list removal

Defcore is removing hosting of the required and flagged test lists[1].
Remove the reference to the lists and replace with the latest RefStack
server generated list.

[1] https://review.openstack.org/#/c/313669/

Change-Id: Ie4edc69a45e4e960f0d19a3741b659ee173e205b

openstack/refstack-client,stackforge/refstack-client,openstack/refstack-client,stackforge/refstack-client