Fix: kill screen on stop-base
Fix: kill screen on stop-base

The screen name can be different from 'stack' in grenade.
Change it to use the global variable.

Change-Id: I57d55b2713dec26140a94c28c4fd373f8989cef7
Blueprint: blueprint/nightly-upgrade-testing

dtroyer/grenade,dtroyer/grenade,openstack-dev/grenade,openstack-dev/grenade