Backport screen_stop to help with Grenade

Back port these commits to stable/havana:
* https://github.com/openstack-dev/devstack/commit/9fc8792b0ac7525b4c353b0a55b8b80eabf76e2a
  https://review.openstack.org/66080
* https://github.com/openstack-dev/devstack/commit/579af5d6786f62008807a473749600e88cea21fc
  https://review.openstack.org/68699

Change-Id: I860cb27a33a4147765b75f0b130782cbc19af9ce